netfilter: ctnetlink: remove dead NAT code

The NAT range to nlattr conversation callbacks and helpers are entirely
dead code and are also useless since there are no NAT ranges in conntrack
context, they are only used for initially selecting a tuple. The final NAT
information is contained in the selected tuples of the conntrack entry.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>