libceph: get rid of ack vs commit

- CEPH_OSD_FLAG_ACK shouldn't be set anymore, so assert on it
- remove support for handling ack replies (OSDs will send ack replies
  only if clients request them)
- drop the "do lingering callbacks under osd->lock" logic from
  handle_reply() -- lreq->lock is sufficient in all three cases

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Sage Weil <sage@redhat.com>