l2tp: remove unused list_head member in l2tp_tunnel

Remove an unused variable in struct l2tp_tunnel which was left behind
by commit c4d48a58 ("l2tp: convert l2tp_tunnel_list to idr").

Signed-off-by: James Chapman <jchapman@katalix.com>
Reviewed-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>