netfilter: conntrack: remove skb argument from nf_ct_refresh

Its not used (and could be NULL), so remove it.
This allows to use nf_ct_refresh in places where we don't have
an skb without having to double-check that skb == NULL would be safe.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>