netfilter: helper: Remove unused function declarations

Commit b1185090 ("netfilter: remove nf_conntrack_helper sysctl and modparam toggles")
leave these unused declarations.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Florian Westphal <fw@strlen.de>