rbd: kill rbd_req_coll and rbd_request

The two remaining callers of rbd_do_request() always pass a null
collection pointer, so the "coll" and "coll_index" parameters are
not needed.  There is no other use of that data structure, so it
can be eliminated.

Deleting them means there is no need to allocate a rbd_request
structure for the callback function.  And since that's the only use
of *that* structure, it too can be eliminated.

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