xen/evtchn: remove spurious barrier

evtchn_unbind_from_user() is called under a lock, so there's no need to
worry about the ordering of unbind_from_irqhandler vs clearing the port
per-user data.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

drivers/xen/evtchn.c

@@ -279,9 +279,6 @@ static void evtchn_unbind_from_user(struct per_user_data *u, int port)
 
 	unbind_from_irqhandler(irq, (void *)(unsigned long)port);
 
-	/* make sure we unbind the irq handler before clearing the port */
-	barrier();
-
 	set_port_user(port, NULL);
 }