[IPV6]: Remove unused declarations in include/net/ip6_route.h.

[IPV6]: Remove unused declarations in include/net/ip6_route.h.

1) Standlaone ip6_null_entry is no longer needed as it is replaced by
   the ip6_null_entry member of ipv6 (instance of struct netns_ipv6) in
   struct net (as a result of Network Namespaces patches).

2) These 3 methods from this same header are not defined anywhere:
   ip6_rt_addr_add(), ip6_rt_addr_del(), rt6_sndmsg()

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>