rbd: kill rbd_rq_fn() and all other related code

Now that the request function has been replaced by one using the new
request management data structures the old one can go away.
Deleting it makes rbd_dev_do_request() no longer needed, and
deleting that makes other functions unneeded, and so on.

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