refactor(core): drop `USBIF.write_blocking()` and simplify FIDO2 keep-alive sending

### core/embed/upymod/modtrezorio/modtrezorio-usb-if.h
@@ -94,29 +94,6 @@ static mp_obj_t mod_trezorio_USBIF_write(mp_obj_t self, mp_obj_t msg) {
 static MP_DEFINE_CONST_FUN_OBJ_2(mod_trezorio_USBIF_write_obj,
                                  mod_trezorio_USBIF_write);
 
-/// def write_blocking(self, msg: AnyBytes, timeout_ms: int) -> int:
-///     """
-///     Sends message using USB interface.
-///     """
-static mp_obj_t mod_trezorio_USBIF_write_blocking(mp_obj_t self, mp_obj_t msg,
-                                                  mp_obj_t timeout_ms) {
-  mp_obj_USBIF_t *o = MP_OBJ_TO_PTR(self);
-  mp_buffer_info_t buf = {0};
-  mp_get_buffer_raise(msg, &buf, MP_BUFFER_READ);
-
-  if (buf.len != USB_PACKET_LEN) {
-    mp_raise_msg(&mp_type_ValueError, MP_ERROR_TEXT("Invalid buffer length"));
-  }
-
-  uint32_t timeout = trezor_obj_get_uint(timeout_ms);
-
-  ssize_t r = syshandle_write_blocking(o->handle, buf.buf, buf.len, timeout);
-
-  return MP_OBJ_NEW_SMALL_INT(r);
-}
-static MP_DEFINE_CONST_FUN_OBJ_3(mod_trezorio_USBIF_write_blocking_obj,
-                                 mod_trezorio_USBIF_write_blocking);
-
 /// def read(self, buf: bytearray, offset: int = 0) -> int:
 ///     """
 ///     Reads message using USB interface
@@ -168,8 +145,6 @@ static const mp_rom_map_elem_t mod_trezorio_USBIF_locals_dict_table[] = {
     {MP_ROM_QSTR(MP_QSTR_iface_num),
      MP_ROM_PTR(&mod_trezorio_USBIF_iface_num_obj)},
     {MP_ROM_QSTR(MP_QSTR_write), MP_ROM_PTR(&mod_trezorio_USBIF_write_obj)},
-    {MP_ROM_QSTR(MP_QSTR_write_blocking),
-     MP_ROM_PTR(&mod_trezorio_USBIF_write_blocking_obj)},
     {MP_ROM_QSTR(MP_QSTR_read), MP_ROM_PTR(&mod_trezorio_USBIF_read_obj)},
     {MP_ROM_QSTR(MP_QSTR_RX_PACKET_LEN), MP_ROM_INT(USB_PACKET_LEN)},
     {MP_ROM_QSTR(MP_QSTR_TX_PACKET_LEN), MP_ROM_INT(USB_PACKET_LEN)},

### core/mocks/generated/trezorio/__init__.pyi
@@ -77,11 +77,6 @@ class USBIF:
         Sends message using USB interface.
         """
 
-    def write_blocking(self, msg: AnyBytes, timeout_ms: int) -> int:
-        """
-        Sends message using USB interface.
-        """
-
     def read(self, buf: bytearray, offset: int = 0) -> int:
         """
         Reads message using USB interface

### core/src/apps/webauthn/fido2.py
@@ -519,33 +519,20 @@ async def send_cmd(cmd: Cmd, iface: HID) -> None:
         seq += 1
 
 
-def send_cmd_sync(cmd: Cmd, iface: HID) -> None:
+def send_keepalive_sync(cid: int, status: int, iface: HID) -> None:
+    cmd = cmd_keepalive(cid, status)
     init_desc = frame_init()
-    cont_desc = frame_cont()
-    offset = 0
-    seq = 0
     datalen = len(cmd.data)
 
     buf, frm = make_struct(init_desc)
     frm.cid = cmd.cid
     frm.cmd = cmd.cmd
     frm.bcnt = datalen
 
-    offset += utils.memcpy(frm.data, 0, cmd.data, offset, datalen)
+    offset = utils.memcpy(frm.data, 0, cmd.data, 0, datalen)
+    assert offset == datalen  # 1-byte payload fits into one USB packet
     iface.write(buf)
 
-    if offset < datalen:
-        frm = overlay_struct(buf, cont_desc)
-
-    while offset < datalen:
-        frm.seq = seq
-        copied = utils.memcpy(frm.data, 0, cmd.data, offset, datalen)
-        offset += copied
-        if copied < _FRAME_CONT_SIZE:
-            frm.data[copied:] = bytearray(_FRAME_CONT_SIZE - copied)
-        iface.write_blocking(buf, 1000)
-        seq += 1
-
 
 async def handle_reports(iface: HID) -> None:
     dialog_mgr = DialogManager(iface)
@@ -571,7 +558,7 @@ def __init__(self, cid: int, iface: HID) -> None:
         self.iface = iface
 
     def __call__(self) -> None:
-        send_cmd_sync(cmd_keepalive(self.cid, _KEEPALIVE_STATUS_PROCESSING), self.iface)
+        send_keepalive_sync(self.cid, _KEEPALIVE_STATUS_PROCESSING, self.iface)
 
 
 async def verify_user(keepalive_callback: KeepaliveCallback) -> bool:
@@ -843,14 +830,14 @@ async def on_confirm(self) -> None:
         cid = self.cid  # local_cache_attribute
 
         self._cred.generate_id()
-        send_cmd_sync(cmd_keepalive(cid, _KEEPALIVE_STATUS_PROCESSING), self.iface)
+        send_keepalive_sync(cid, _KEEPALIVE_STATUS_PROCESSING, self.iface)
         response_data = _cbor_make_credential_sign(
             self._client_data_hash, self._cred, self._user_verification
         )
 
         cmd = Cmd(cid, _CMD_CBOR, bytes([_ERR_NONE]) + response_data)
         if self._resident:
-            send_cmd_sync(cmd_keepalive(cid, _KEEPALIVE_STATUS_PROCESSING), self.iface)
+            send_keepalive_sync(cid, _KEEPALIVE_STATUS_PROCESSING, self.iface)
             if not store_resident_credential(self._cred):
                 cmd = cbor_error(cid, _ERR_KEY_STORE_FULL)
         await send_cmd(cmd, self.iface)
@@ -911,7 +898,7 @@ async def on_confirm(self) -> None:
 
         assert self._selected_cred is not None
         try:
-            send_cmd_sync(cmd_keepalive(cid, _KEEPALIVE_STATUS_PROCESSING), self.iface)
+            send_keepalive_sync(cid, _KEEPALIVE_STATUS_PROCESSING, self.iface)
             response_data = cbor_get_assertion_sign(
                 self._client_data_hash,
                 self._selected_cred.rp_id_hash,