mlxsw: spectrum_router: Start using new IPv4 route notifications

With the new notifications mlxsw does not need to handle identical
routes itself, as this is taken care of by the core IPv4 code.

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

Convert mlxsw to use the new IPv4 route notifications and simplify the
code.

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