libceph: Remove unused ceph_osdc_watch_check

ceph_osdc_watch_check() has been unused since it was added in commit
b07d3c4b ("libceph: support for checking on status of watch")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>