netfilter: Clean up unnecessary #ifdef

If CONFIG_NETFILTER_INGRESS is not enabled, nf_ingress() becomes a no-op
because it solely contains an if-clause calling nf_hook_ingress_active(),
for which an empty inline stub exists in <linux/netfilter_ingress.h>.

All the symbols used in the if-clause's body are still available even if
CONFIG_NETFILTER_INGRESS is not enabled.

The additional "#ifdef CONFIG_NETFILTER_INGRESS" in nf_ingress() is thus
unnecessary, so drop it.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>