netfilter: conntrack: destroy kmemcache on module removal

I forgot to move the kmem_cache_destroy into the exit path.

Fixes: 0c5366b3 ("netfilter: conntrack: use single slab cache)
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>