mlxsw: spectrum_router: Start using new IPv6 route notifications

With the new notifications mlxsw does not need to handle identical
routes itself, as this is taken care of by the core IPv6 code.

Instead, mlxsw only needs to take care of inserting and removing routes
from the device.

Convert mlxsw to use the new IPv6 route notifications and simplify the
code.

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