[IPV6]: Remove struct pol_chain.

Struct pol_chain has existed since at least the 2.2 kernel, but isn't used
anymore. As the IPv6 policy routing is implemented in a totally different
way in the current kernel, just get rid of it.

Signed-off-by: Ville Nuorvala <vnuorval@tcs.hut.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>