This is a re-do of 57d334a13d (#13365),
which was backed out in 12abd72497 (#13501).
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.
