Nothing uses this now, so we can remove the dead code, and clean up the
API.
Since we're changing the shape of the return value anyway, we take the
opportunity to give the method a better name.
This endpoint isn't much use for its intended purpose if you first need to get
yourself an admin's auth token.
I've restricted it to the `/_synapse/admin` path to make it a bit easier to
lock down for those concerned about exposing this information. I don't imagine
anyone is using it in anger currently.
