Stop depending on `room_id` to be returned for children state in the hierarchy response. (#12991)

The `room_id` field was removed from MSC2946 before
it was accepted. It was initially kept for backwards compatibility
and should be removed now that the stable form of the API
is used.

This change only stops Synapse from validating that it is returned,
a future PR will remove returning it as part of the response.