The nat module already exposes a few functions to the conntrack core. Move the nat extension destroy hook to it. After this, no conntrack extension needs a destroy hook. 'struct nf_ct_ext_type' and the register/unregister api can be removed in a followup patch. Signed-off-by: Florian Westphal <fw@strlen.de> Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>