Remove definition of Event for /notifications (#3525)

The documentation for the `/notifications` API had its own special definition
of what an Event was, which was used nowhere else.

The common definition isn't perfect, but it *is* common, so it gives us a
better starting place for improvement.

data/api/client-server/notifications.yaml

@@ -113,7 +113,7 @@ paths:
                       title: Event
                       description: The Event object for the event that triggered the notification.
                       allOf:
-                        - "$ref": "definitions/event.yaml"
+                        - $ref: ../../event-schemas/schema/core-event-schema/sync_room_event.yaml
                     profile_tag:
                       type: string
                       description: The profile tag of the rule that matched this event.