[NETFILTER]: cleanup IPv6 Netfilter Kconfig

This removes linux 2.4 configs in comments as TODO lists.
And this also move the entry of nf_conntrack to top like IPv4 Netfilter
Kconfig.

Based on original patch by Krzysztof Piotr Oledzki <ole@ans.pl>.

Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>