net: phy: remove stub for mdiobus_register_board_info

net: phy: remove stub for mdiobus_register_board_info

The functionality of mdiobus_register_board_info() typically isn't
optional for the caller. Therefore remove the stub.

Note: Currently we have only one caller of mdiobus_register_board_info(),
in a DSA/PHYLINK context. Therefore CONFIG_MDIO_DEVICE is selected anyway.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://patch.msgid.link/410a2222-c4e8-45b0-9091-d49674caeb00@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>