rbd: kill __rbd_remove()

The function __rbd_remove() is used in two spots, and it's fairly
simple.  It combines cleanup of part of the ceph-side state as well
as cleaning up the Linux-side state.  Just open code it in the two
callers and eliminate the function.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>