net: phy: vitesse: remove duplicate support for VSC8574

A more featureful support for VSC8574 was recently added to the
Microsemi (mscc.c) driver. I checked that features supported in the
Vitesse driver are also supported in the Microsemi driver.

Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>