Revert "support admin_email config and pass through into blocking errors, return AuthError in all cases"

This reverts commit 0d43f991.

This is the first tranche of support for room versioning. It includes:
 * setting the default room version in the config file
 * new room_version param on the createRoom API
 * storing the version of newly-created rooms in the m.room.create event
 * fishing the version of existing rooms out of the m.room.create event

This reverts commit ec716a35.

The field is never read from, and all the opportunities given to populate it are not utilized. It should be very safe to remove this.

Because it was complicated and annoyed me. I suspect this will be more
efficient too.