[NETFILTER]: nf_nat: kill global 'destroy' operation

This kills the global 'destroy' operation which was used by NAT.
Instead it uses the extension infrastructure so that multiple
extensions can register own operations.

Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>