netfilter: conntrack: remove obsolete sysctl (nf_conntrack_events_retry_timeout)

This entry has been removed in commit 9500507c.

Fixes: 9500507c ("netfilter: conntrack: remove timer from ecache extension")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>