libceph: drop msg->ack_stamp field

It is set in process_ack() but never used.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

include/linux/ceph/messenger.h

@@ -235,7 +235,6 @@ struct ceph_msg {
 	bool more_to_follow;
 	bool needs_out_seq;
 	int front_alloc_len;
-	unsigned long ack_stamp;        /* tx: when we were acked */
 
 	struct ceph_msgpool *pool;
 };

net/ceph/messenger.c

@@ -2279,7 +2279,6 @@ static void process_ack(struct ceph_connection *con)
 			break;
 		dout("got ack for seq %llu type %d at %p\n", seq,
 		     le16_to_cpu(m->hdr.type), m);
-		m->ack_stamp = jiffies;
 		ceph_msg_remove(m);
 	}