Allow shutdowns of rooms using the admin api without replacement rooms

[miterion]

Description:
Currently, the /_synapse/admin/v1/shutdown_room/{room_id} admin api moves all users to a new or existing room upon calling.
It is not possible to avoid this by simply omitting the new_room_user_id parameters, as can be done when using the /_synapse/admin/v1/rooms/<room_id>/delete endpoint.

Since the underlying code (RoomShutdownHandler) already supports the omission of new_room_user_id I think it is sufficient to just remove the restrictions currently in place and update the documentation to fix my issue.

[dklimpel]

Why do you not switch to the new API /delete?
There are all options what you looking for.

[miterion]

/debug does not work for me probably due to bug in either synapse or my configuration. I looked for alternatives and found the /shutdown endpoint and noticed the discrepancy between their parameters.
If it is intended to drop the /shutdown endpoint then please close this issue.

[anoadragon453]

@miterion We had a discussion in #synapse-dev from this issue, and yes, we've decided to do exactly that: #8663 😉