Avoid using other keys next to $ref

There are very limited exceptions where OpenAPI spec allows other keys
next to $ref.

data/api/client-server/key_backup.yaml

@@ -718,7 +718,6 @@ paths:
           content:
             application/json:
               schema:
-                type: object
                 $ref: definitions/room_key_backup.yaml
         "404":
           description: The backup was not found.