proto: remove orphan SendToRouteResponse and outgoing_chan_id fields

Drop the now-orphan routerrpc.SendToRouteResponse message that was only
referenced by the deleted routerrpc.SendToRoute RPC.

Also remove the deprecated outgoing_chan_id field from
lnrpc.QueryRoutesRequest (tag 14) and routerrpc.SendPaymentRequest
(tag 8); their tag numbers are now reserved. Callers must use the
multi-channel outgoing_chan_ids field introduced in 0.20.

Drop the compat fallback in router_backend.go that previously consumed
the field, and regenerate the affected pb.go/swagger artifacts.

Author: erickcestari

lnrpc/lightning.pb.go

@@ -3141,11 +3141,7 @@ message QueryRoutesRequest {
     */
     map<uint64, bytes> dest_custom_records = 13;
 
-    /*
-    Deprecated, use outgoing_chan_ids. The channel id of the channel that must
-    be taken to the first hop. If zero, any channel may be used.
-    */
-    uint64 outgoing_chan_id = 14 [jstype = JS_STRING, deprecated = true];
+    reserved 14;
 
     /*
     The pubkey of the last hop of the route. If empty, any hop may be used.

lnrpc/lightning.proto

@@ -1357,14 +1357,6 @@
             "required": false,
             "type": "string"
           },
-          {
-            "name": "outgoing_chan_id",
-            "description": "Deprecated, use outgoing_chan_ids. The channel id of the channel that must\nbe taken to the first hop. If zero, any channel may be used.",
-            "in": "query",
-            "required": false,
-            "type": "string",
-            "format": "uint64"
-          },
           {
             "name": "last_hop_pubkey",
             "description": "The pubkey of the last hop of the route. If empty, any hop may be used.",
@@ -1538,11 +1530,6 @@
                   },
                   "description": "An optional field that can be used to pass an arbitrary set of TLV records\nto a peer which understands the new records. This can be used to pass\napplication specific data during the payment attempt. If the destination\ndoes not support the specified records, an error will be returned.\nRecord types are required to be in the custom range \u003e= 65536. When using\nREST, the values must be encoded as base64."
                 },
-                "outgoing_chan_id": {
-                  "type": "string",
-                  "format": "uint64",
-                  "description": "Deprecated, use outgoing_chan_ids. The channel id of the channel that must\nbe taken to the first hop. If zero, any channel may be used."
-                },
                 "last_hop_pubkey": {
                   "type": "string",
                   "format": "byte",