[NETFILTER] ip6tables: remove unused definitions

These definitions ware used for only internal use in kernel <= 2.6.13,
which had not introduced the unified parser of IPv6 extension header yet.

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>