ipvs: use nf_ct_kill helper

Once timer is removed from nf_conn struct we cannot open-code
the removal sequence anymore.

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>