Remove event ID usage when checking if new room

The event ID is changing, so we can no longer get the domain from it. On
the other hand, the check is unnecessary.

synapse/handlers/room_member.py

@@ -940,7 +940,8 @@ class RoomMemberHandler(object):
         # first member event?
         create_event_id = current_state_ids.get(("m.room.create", ""))
         if len(current_state_ids) == 1 and create_event_id:
-            defer.returnValue(self.hs.is_mine_id(create_event_id))
+            # We can only get here if we're in the process of creating the room
+            defer.returnValue(True)
 
         for etype, state_key in current_state_ids:
             if etype != EventTypes.Member or not self.hs.is_mine_id(state_key):