ipset: remove unused function __ip_set_get_netlink

There are no in-tree callers.

Signed-off-by: Aaron Conole <aconole@bytheb.org>
Acked-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>