netfilter: remove hook owner refcounting

since commit 8405a8ff ("netfilter: nf_qeueue: Drop queue entries on
nf_unregister_hook") all pending queued entries are discarded.

So we can simply remove all of the owner handling -- when module is
removed it also needs to unregister all its hooks.

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