Update room_event_filter.yaml to use the OpenAPI allOf definition

This is just maintenance.

api/client-server/definitions/room_event_filter.yaml

@@ -13,8 +13,9 @@
 # limitations under the License.
 allOf:
 - $ref: event_filter.yaml
-title: RoomEventFilter
+- type: object
-properties:
+  title: RoomEventFilter
+  properties:
     not_rooms:
       description: A list of room IDs to exclude. If this list is absent then no rooms
         are excluded. A matching room will be excluded even if it is listed in the ``'rooms'``
@@ -32,4 +33,3 @@ properties:
       type: boolean
     description: If ``true``, includes only events with a url key in their content. If
       ``false``, excludes those events.
-type: object