netfilter: conntrack: Remove unused function declarations

Commit 1015c3de ("netfilter: conntrack: remove extension register api")
leave nf_conntrack_acct_fini() and nf_conntrack_labels_init() unused, remove it.
And commit a0ae2562 ("netfilter: conntrack: remove l3proto abstraction")
leave behind nf_ct_l3proto_try_module_get() and nf_ct_l3proto_module_put().

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