netfilter: nf_nat: remove obsolete code from nf_nat_icmp_reply_translation()

The inner tuple that is extracted from the packet is unused. The code also
doesn't have any useful side-effects like verifying the packet does contain
enough data to extract the inner tuple since conntrack already does the
same, so remove it.

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