Remove the unstable event field for /send_join per MSC3083. (#12395)

This was missed when initially stabilising room version 8 and was
left in as a compatibility shim. Most homeservers have upgraded
to a version which expects the proper field name, and the failure
mode is reasonable (a user on an older server may have to attempt
joining the room twice with an obscure error message the first time).

changelog.d/12395.misc

@@ -0,0 +1 @@
+Remove an unstable identifier from [MSC3083](https://github.com/matrix-org/matrix-doc/pull/3083).

synapse/federation/federation_server.py

@@ -687,8 +687,6 @@ async def on_send_join_request(
         time_now = self._clock.time_msec()
         event_json = event.get_pdu_json(time_now)
         resp = {
-            # TODO Remove the unstable prefix when servers have updated.
-            "org.matrix.msc3083.v2.event": event_json,
             "event": event_json,
             "state": [p.get_pdu_json(time_now) for p in state_events],
             "auth_chain": [p.get_pdu_json(time_now) for p in auth_chain_events],

synapse/federation/transport/client.py

@@ -1380,16 +1380,6 @@ def __init__(self, room_version: RoomVersion, v1_api: bool):
                 prefix + "auth_chain.item",
                 use_float=True,
             ),
-            # TODO Remove the unstable prefix when servers have updated.
-            #
-            # By re-using the same event dictionary this will cause the parsing of
-            # org.matrix.msc3083.v2.event and event to stomp over each other.
-            # Generally this should be fine.
-            ijson.kvitems_coro(
-                _event_parser(self._response.event_dict),
-                prefix + "org.matrix.msc3083.v2.event",
-                use_float=True,
-            ),
             ijson.kvitems_coro(
                 _event_parser(self._response.event_dict),
                 prefix + "event",