netfilter: nf_nat: remove warn_if_extra_mangle

In net/ipv4/netfilter/nf_nat_rule.c, the function warn_if_extra_mangle was added
in commit 5b1158e9

 (2006-12-02). I have a DNAT
target in the OUTPUT chain than changes connections with dst 2.0.0.1 to another
address which I'll substitute with 66.102.9.99 below.

On every boot I get the following message:
[  146.252505] NAT: no longer support implicit source local NAT
[  146.252517] NAT: packet src 66.102.9.99 -> dst 2.0.0.1

As far as I can tell from reading the function doing this, it should warn if the
source IP for the route to 66.102.9.99 is different from 2.0.0.1 but that is not
the case. It doesn't make sense to check the DNAT target against the local route
source.

Either the function should be changed to correctly check the route, or it should
be removed entirely as it's been nearly 2 years since it was added.

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Signed-off-by: Patrick McHardy <kaber@trash.net>