zephyr: remove Kconfig option CONFIG_USB

USB device support configuration is revised,
CONFIG_USB option is deprecated.
Only USB_DEVICE_STACK option should be used to
enable USB device support.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>
Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>