l2tp: cleanup l2tp_tunnel_delete calls

l2tp: cleanup l2tp_tunnel_delete calls

[ Upstream commit 4dc12ffeaeac939097a3f55c881d3dc3523dff0c ]

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>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>