hash_v6 is used by both nftables and ip6tables, so depend on
IP6_NF_IPTABLES is not properly.

Actually, it only parses ipv6hdr and computes a hash value, so
even if IPV6 is disabled, there's no side effect too, remove it.
Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>