net: mvneta: Don't register MDIO bus

This MDIO bus is now handled by a proper mvmdio DM driver. Remove it
from mvneta.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>