net: bcmgenet: Drop legacy MDIO code

Now that we have fully migrated to the mdio-bcm-unimac driver, drop the
legacy MDIO bus code which did duplicate a fair amount of code.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>