ipv6: Remove old route notifications and convert listeners

Now that mlxsw is converted to use the new FIB notifications it is
possible to delete the old ones and use the new replace / append /
delete notifications.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>