Don't allow server admins to bypass user_may_join_room callback

It's mostly used to track accepting invites, not actually blocking joins,
so it should be able to track everyone.

synapse/handlers/room_member.py

@@ -1000,9 +1000,6 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
                 # allow the server notices mxid to join rooms
                 bypass_spam_checker = True
 
-            else:
-                bypass_spam_checker = await self.auth.is_server_admin(requester)
-
             inviter = await self._get_inviter(target.to_string(), room_id)
             if (
                 not bypass_spam_checker