If you disable IGPU (or select force PCIe slot 1) in Bios then uninstall and reinstall the Radeon software it should remove the Vega 8 from the software completely and only the RX560 will be showing. Don't forget to reassign any extra RAM you have dedicated to onboard graphics back to the system while there.
Don't forget to plug directly into the Video card and not the motherboard.
PS: you may have to go into Device Manager and right click on Display Adapters and uninstall the driver manually before reinstalling the Radeon Software.
