l2tp: cleanup l2tp_tunnel_delete calls

l2tp_tunnel_delete does not return anything since commit 62b982eeb458
("l2tp: fix race condition in l2tp_tunnel_delete").  But call sites of
l2tp_tunnel_delete still do casts to void to avoid unused return value
warnings.

Kill these now useless casts.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Sabrina Dubroca <sd@queasysnail.net>
Cc: Guillaume Nault <g.nault@alphalink.fr>
Cc: David S. Miller <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Acked-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/l2tp/l2tp_netlink.c b/net/l2tp/l2tp_netlink.c
index f517942..f04fb34 100644
--- a/net/l2tp/l2tp_netlink.c
+++ b/net/l2tp/l2tp_netlink.c
@@ -282,7 +282,7 @@
 	l2tp_tunnel_notify(&l2tp_nl_family, info,
 			   tunnel, L2TP_CMD_TUNNEL_DELETE);
 
-	(void) l2tp_tunnel_delete(tunnel);
+	l2tp_tunnel_delete(tunnel);
 
 	l2tp_tunnel_dec_refcount(tunnel);