ipv4/arp.c:arp_process(): remove bogus #ifdef mess

The #ifdef's in arp_process() were not only a mess, they were also wrong
in the CONFIG_NET_ETHERNET=n and (CONFIG_NETDEV_1000=y or
CONFIG_NETDEV_10000=y) cases.

Since they are not required this patch removes them.

Also removed are some #ifdef's around #include's that caused compile
errors after this change.

Signed-off-by: Adrian Bunk <bunk@kernel.org>