[IPV6]: remove more unused IPV6_AUTHHDR things.

Remove two more unused IPV6_AUTHHDR option things, 
which I failed to remove them last time,
plus, mark IPV6_AUTHHDR obsolete.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>