routerrpc: remove deprecated outgoing_chan_id field handling

diff --git a/lnrpc/routerrpc/router_backend.go b/lnrpc/routerrpc/router_backend.go
index 3bb4d82..a639138 100644
--- a/lnrpc/routerrpc/router_backend.go
+++ b/lnrpc/routerrpc/router_backend.go
@@ -447,19 +447,16 @@ func (r *RouterBackend) parseQueryRoutesRequest(in *lnrpc.QueryRoutesRequest) (
 		BlindedPaymentPathSet: blindedPathSet,
 	}
 
-	// We set the outgoing channel restrictions if the user provides a
-	// list of channel ids. We also handle the case where the user
-	// provides the deprecated `OutgoingChanId` field.
-	switch {
-	case len(in.OutgoingChanIds) > 0 && in.OutgoingChanId != 0:
-		return nil, errors.New("outgoing_chan_id and " +
-			"outgoing_chan_ids cannot both be set")
+	// The deprecated single outgoing_chan_id field is no longer
+	// honored. Reject requests that still set it so that callers do
+	// not silently get an unrestricted route.
+	if in.OutgoingChanId != 0 {
+		return nil, errors.New("outgoing_chan_id is deprecated, " +
+			"use outgoing_chan_ids")
+	}
 
-	case len(in.OutgoingChanIds) > 0:
+	if len(in.OutgoingChanIds) > 0 {
 		restrictions.OutgoingChannelIDs = in.OutgoingChanIds
-
-	case in.OutgoingChanId != 0:
-		restrictions.OutgoingChannelIDs = []uint64{in.OutgoingChanId}
 	}
 
 	// Pass along a last hop restriction if specified.
@@ -880,21 +877,16 @@ func (r *RouterBackend) extractIntentFromSendRequest(
 	}
 	payIntent.TimePref = rpcPayReq.TimePref
 
-	// Pass along restrictions on the outgoing channels that may be used.
-	payIntent.OutgoingChannelIDs = rpcPayReq.OutgoingChanIds
-
-	// Add the deprecated single outgoing channel restriction if present.
+	// The deprecated single outgoing_chan_id field is no longer
+	// honored. Reject requests that still set it so that callers do
+	// not silently get an unrestricted payment.
 	if rpcPayReq.OutgoingChanId != 0 {
-		if payIntent.OutgoingChannelIDs != nil {
-			return nil, errors.New("outgoing_chan_id and " +
-				"outgoing_chan_ids are mutually exclusive")
-		}
-
-		payIntent.OutgoingChannelIDs = append(
-			payIntent.OutgoingChannelIDs, rpcPayReq.OutgoingChanId,
-		)
+		return nil, errors.New("outgoing_chan_id is deprecated, " +
+			"use outgoing_chan_ids")
 	}
 
+	payIntent.OutgoingChannelIDs = rpcPayReq.OutgoingChanIds
+
 	// Pass along a last hop restriction if specified.
 	if len(rpcPayReq.LastHopPubkey) > 0 {
 		lastHop, err := route.NewVertexFromBytes(
diff --git a/lnrpc/routerrpc/router_backend_test.go b/lnrpc/routerrpc/router_backend_test.go
index e572f80..e7ca056 100644
--- a/lnrpc/routerrpc/router_backend_test.go
+++ b/lnrpc/routerrpc/router_backend_test.go
@@ -33,39 +33,28 @@ var (
 	node2 = route.Vertex{11}
 )
 
-var (
-	singleChanID = "singleChanID"
-	multiChanID  = "multiChanID"
-	bothChanIds  = "bothChanIds"
-)
-
 // TestQueryRoutes asserts that query routes rpc parameters are properly parsed
 // and passed onto path finding.
 func TestQueryRoutes(t *testing.T) {
 	t.Run("no mission control", func(t *testing.T) {
-		testQueryRoutes(t, false, false, true, singleChanID)
+		testQueryRoutes(t, false, false, true)
 	})
 	t.Run("no mission control and msat", func(t *testing.T) {
-		testQueryRoutes(t, false, true, true, singleChanID)
+		testQueryRoutes(t, false, true, true)
 	})
 	t.Run("with mission control", func(t *testing.T) {
-		testQueryRoutes(t, true, false, true, singleChanID)
+		testQueryRoutes(t, true, false, true)
 	})
 	t.Run("no mission control bad cltv limit", func(t *testing.T) {
-		testQueryRoutes(t, false, false, false, singleChanID)
+		testQueryRoutes(t, false, false, false)
 	})
-
-	t.Run("both outgoing chan id and chan ids", func(t *testing.T) {
-		testQueryRoutes(t, true, false, true, bothChanIds)
-	})
-
 	t.Run("multiple outgoing chan ids", func(t *testing.T) {
-		testQueryRoutes(t, false, true, true, multiChanID)
+		testQueryRoutes(t, false, true, true)
 	})
 }
 
 func testQueryRoutes(t *testing.T, useMissionControl bool, useMsat bool,
-	setTimelock bool, outgoingChanConfig string) {
+	setTimelock bool) {
 
 	ignoreNodeBytes, err := hex.DecodeString(ignoreNodeKey)
 	if err != nil {
@@ -82,7 +71,6 @@ func testQueryRoutes(t *testing.T, useMissionControl bool, useMsat bool,
 
 	var (
 		lastHop         = route.Vertex{64}
-		outgoingChan    = uint64(383322)
 		outgoingChanIds = []uint64{383322, 383323}
 	)
 
@@ -137,17 +125,7 @@ func testQueryRoutes(t *testing.T, useMissionControl bool, useMsat bool,
 		}
 	}
 
-	switch outgoingChanConfig {
-	case singleChanID:
-		request.OutgoingChanId = outgoingChan
-
-	case multiChanID:
-		request.OutgoingChanIds = outgoingChanIds
-
-	case bothChanIds:
-		request.OutgoingChanId = outgoingChan
-		request.OutgoingChanIds = outgoingChanIds
-	}
+	request.OutgoingChanIds = outgoingChanIds
 
 	findRoute := func(req *routing.RouteRequest) (*route.Route, float64,
 		error) {
@@ -190,19 +168,9 @@ func testQueryRoutes(t *testing.T, useMissionControl bool, useMsat bool,
 			t.Fatal("unexpected last hop")
 		}
 
-		switch outgoingChanConfig {
-		case singleChanID:
-			require.Equal(
-				t, restrictions.OutgoingChannelIDs,
-				[]uint64{outgoingChan},
-			)
-
-		case multiChanID:
-			require.Equal(
-				t, restrictions.OutgoingChannelIDs,
-				outgoingChanIds,
-			)
-		}
+		require.Equal(
+			t, restrictions.OutgoingChannelIDs, outgoingChanIds,
+		)
 
 		if !restrictions.DestFeatures.HasFeature(lnwire.MPPOptional) {
 			t.Fatal("unexpected dest features")
@@ -265,13 +233,6 @@ func testQueryRoutes(t *testing.T, useMissionControl bool, useMsat bool,
 
 	resp, err := backend.QueryRoutes(t.Context(), request)
 
-	// If we're using both OutgoingChanId and OutgoingChanIds, we should get
-	// an error.
-	if outgoingChanConfig == bothChanIds {
-		require.Error(t, err)
-		return
-	}
-
 	// If no MaxTotalTimelock was set for the QueryRoutes request, make
 	// sure an error was returned.
 	if !setTimelock {
@@ -573,17 +534,6 @@ func TestExtractIntentFromSendRequest(t *testing.T) {
 			valid:            false,
 			expectedErrorMsg: "time preference out of range",
 		},
-		{
-			name:    "Outgoing channel exclusivity violation",
-			backend: &RouterBackend{},
-			sendReq: &SendPaymentRequest{
-				OutgoingChanId:  38484,
-				OutgoingChanIds: []uint64{383322},
-			},
-			valid: false,
-			expectedErrorMsg: "outgoing_chan_id and " +
-				"outgoing_chan_ids are mutually exclusive",
-		},
 		{
 			name:    "Invalid last hop pubkey length",
 			backend: &RouterBackend{},