Remove redundant store_room call (#6979)

`_process_received_pdu` is only called by `on_receive_pdu`, which ignores any
events for unknown rooms, so this is redundant.

changelog.d/6979.misc

@@ -0,0 +1 @@
+Remove redundant `store_room` call from `FederationHandler._process_received_pdu`.

synapse/handlers/federation.py

@@ -41,7 +41,6 @@
  FederationDeniedError,
  FederationError,
  RequestSendFailed,
- StoreError,
  SynapseError,
 )
 from synapse.api.room_versions import KNOWN_ROOM_VERSIONS, RoomVersion, RoomVersions
@@ -707,28 +706,6 @@ async def _process_received_pdu(
  except AuthError as e:
  raise FederationError("ERROR", e.code, e.msg, affected=event.event_id)
 
- room = await self.store.get_room(room_id)
-
- if not room:
- try:
- prev_state_ids = await context.get_prev_state_ids()
- create_event = await self.store.get_event(
- prev_state_ids[(EventTypes.Create, "")]
- )
-
- room_version_id = create_event.content.get(
- "room_version", RoomVersions.V1.identifier
- )
-
- await self.store.store_room(
- room_id=room_id,
- room_creator_user_id="",
- is_public=False,
- room_version=KNOWN_ROOM_VERSIONS[room_version_id],
- )
- except StoreError:
- logger.exception("Failed to store room.")
-
  if event.type == EventTypes.Member:
  if event.membership == Membership.JOIN:
  # Only fire user_joined_room if the user has acutally