xen/pvcalls-back: Remove redundant 'flush_workqueue()' calls

'destroy_workqueue()' already drains the queue before destroying it, so
there is no need to flush it explicitly.

Remove the redundant 'flush_workqueue()' calls.

This was generated with coccinelle:

@@
expression E;
@@
- 	flush_workqueue(E);
	destroy_workqueue(E);
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Link: https://lore.kernel.org/r/2d6c2e031e4aa2acf2ac4e0bbbc17cfdcc8dbee2.1634236560.git.christophe.jaillet@wanadoo.frSigned-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/pvcalls-back.c

@@ -465,7 +465,6 @@ static int pvcalls_back_release_passive(struct xenbus_device *dev,
 		write_unlock_bh(&mappass->sock->sk->sk_callback_lock);
 	}
 	sock_release(mappass->sock);
-	flush_workqueue(mappass->wq);
 	destroy_workqueue(mappass->wq);
 	kfree(mappass);