netfilter: nf_ct_helper: use nf_ct_iterate_destroy to unlink helper objs

When we unlink the helper objects, we will iterate the nf_conntrack_hash,
iterate the unconfirmed list, handle the hash resize situation, etc.

Actually this logic is same as the nf_ct_iterate_destroy, so we can use
it to remove these copy & paste code.

Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>