Remove the unspecced room_id field in the /hierarchy response. (#13506)

This is a re-do of 57d334a1 (#13365),
which was backed out in 12abd724 (#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.