Remove unused room_alias field from /createRoom response (#15093)

* Change `create_room` return type

* Don't return room alias from /createRoom

* Update other callsites

* Fix up mypy complaints

It looks like new_room_user_id is None iff new_room_id is None. It's a
shame we haven't expressed this in a way that mypy can understand.

* Changelog

tests/test_federation.py

@@ -47,7 +47,7 @@ class MessageAcceptTests(unittest.HomeserverTestCase):
             room_creator.create_room(
                 our_user, room_creator._presets_dict["public_chat"], ratelimit=False
             )
-        )[0]["room_id"]
+        )[0]
 
         self.store = self.hs.get_datastores().main