netfilter: conntrack: remove empty pernet fini stubs

after moving sysctl handling into single place, the init functions
can't fail anymore and some of the fini functions are empty.

Remove them and change return type to void.
This also simplifies error unwinding in conntrack module init path.

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