boards: remove ECM/EEM from default configuration

The board hardware has no network interfaces, although the ECM/EEM class
implementation can provide Ethernet-like functionality and export it to
the host, this is no reason to default to a specific USB class
implementation.We do not make this kind of configuration for other
boards that have a USB device controller. Also, it may not be what a
user expects when using these boards with networking and a USB stack.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>