[IPV4]: Remove unsupported DNAT (RTCF_NAT and RTCF_NAT) in IPV4

- The DNAT (Destination NAT) is not implemented in IPV4.

- This patch remove the code which checks these flags
in net/ipv4/arp.c and net/ipv4/route.c.

The RTCF_NAT and RTCF_NAT should stay in the header (linux/in_route.h)
because they are used in DECnet.

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>