mlxsw: spectrum_router: Eliminate dead code

Since the driver no longer maintains a list of identical routes there is
no route to promote when a route is deleted.

Remove that code that took care of it.

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