Remove the unspecced room_id field in the /hierarchy response. (#13365)

The `room_id` field represented the parent space for each room
and was made redundant by changes in the API shape where the
`children_state` is now nested underneath each `room`.

The room ID of each child is in the `state_key` field and is still
available.

synapse/handlers/room_summary.py

@@ -452,7 +452,6 @@ class RoomSummaryHandler:
                 "type": e.type,
                 "state_key": e.state_key,
                 "content": e.content,
-                "room_id": e.room_id,
                 "sender": e.sender,
                 "origin_server_ts": e.origin_server_ts,
             }