libceph: drop ceph_con_get/put helpers and nref member

These are no longer used.  Every ceph_connection instance is embedded in
another structure, and refcounts manipulated via the get/put ops.

Signed-off-by: Sage Weil <sage@inktank.com>

include/linux/ceph/messenger.h

@@ -135,7 +135,6 @@ struct ceph_msg_pos {
  */
 struct ceph_connection {
 	void *private;
-	atomic_t nref;
 
 	const struct ceph_connection_operations *ops;