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 <[e-mail]>
Acked-by: Stefano Stabellini <[e-mail]>
Link: https://lore.kernel.org/r/2d6c2e031e[e-mail]
Signed-off-by: Boris Ostrovsky <[e-mail]>

drivers/xen/pvcalls-back.c

···
 		write_unlock_bh(&mappass->sock->sk->sk_callback_lock);
 	}
 	sock_release(mappass->sock);
-	flush_workqueue(mappass->wq);
 	destroy_workqueue(mappass->wq);
 	kfree(mappass);