[NETFILTER] nat: remove bogus structure member

When 'rustynat' was merged in 2.6.12, the use of the "helper" pointer of
struct ipt_nat_info was obsoleted, but the pointer not removed from the
struct.

This patch removes the pointer, thereby yet again shrinking struct
ip_conntrack.

Discovered-by: Rusty Russell <rusty@netfilter.org>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>