Platform/RaspberryPi4: remove ASIX 88772b driver

The Raspberry Pi4 has gigabit ethernet builtin, for which we now
have a UEFI driver. So no need for the ASIX 88772b driver.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Andrei Warkentin <andrey.warkentin@gmail.com>
Reviewed-by: Samer El-Haj-Mahmoud <Samer.El-Haj-Mahmoud@arm.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>