netfilter: cttimeout: use nf_ct_iterate_cleanup_net to unlink timeout objs

Similar to nf_conntrack_helper, we can use nf_ct_iterare_cleanup_net to
remove these copy & paste code.

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