ipv6: remove unused function ipv6_inherit_linklocal()

It is no longer used after commit e837735e
(ip6_tunnel: ensure to always have a link local address).

Cc: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>