rbd: get rid of rbd_req_sync_exec()

Get rid rbd_req_sync_exec() because it is no longer used.  That
eliminates the last use of rbd_req_sync_op(), so get rid of that
too.  And finally, that leaves rbd_do_request() unreferenced, so get
rid of that.

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