netfilter: conntrack: remove nf_conntrack_helper documentation

This toggle has been already remove by b1185090 ("netfilter: remove
nf_conntrack_helper sysctl and modparam toggles").

Remove the documentation entry for this toggle too.

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