l2tp: remove l2tp_session_find()

commit 55a3ce3b9d98f752df9e2cfb1cba7e715522428a upstream.

This function isn't used anymore.

Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Giuliano Procida <gprocida@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>