l2tp: drop the now unused l2tp_tunnel_get_session

All users of l2tp_tunnel_get_session are now gone so it can be
removed.

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