The last user of ip_forward_set, lxc_ip_forward_on and
lxc_ip_forward_off was in 2009:

commit 92d38522
Author: Daniel Lezcano <dlezcano@fr.ibm.com>
Date:   Thu Oct 22 15:33:40 2009 +0200

    remove test directory

These functions are not called anymore.
Signed-off-by: Marcos Paulo de Souza <marcos.souza.org@gmail.com>