net: dsa: mv88e6xxx: Remove legacy probe support

Remove the legacy method of probing the mv88e6xxx driver, now that all
the mainline boards have been converted to use mdio based probing for
a number of cycles.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>