xen/pvcalls: remove set but not used variable 'intf'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/xen/pvcalls-back.c: In function 'pvcalls_sk_state_change':
drivers/xen/pvcalls-back.c:286:28: warning:
 variable 'intf' set but not used [-Wunused-but-set-variable]

It not used since e6587cdb ("pvcalls-back: set -ENOTCONN in
pvcalls_conn_back_read")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/pvcalls-back.c

@@ -283,12 +283,10 @@ static int pvcalls_back_socket(struct xenbus_device *dev,
 static void pvcalls_sk_state_change(struct sock *sock)
 {
 	struct sock_mapping *map = sock->sk_user_data;
-	struct pvcalls_data_intf *intf;
 
 	if (map == NULL)
 		return;
 
-	intf = map->ring;
 	atomic_inc(&map->read);
 	notify_remote_via_irq(map->irq);
 }