netfilter: nf_nat_ipv4: remove obsolete EXPORT_SYMBOL

There are no external callers anymore, previous change just
forgot to also remove the EXPORT_SYMBOL().

Fixes: 9971a514

 ("netfilter: nf_nat: add nat type hooks to nat core")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>