usb: device: remove loopback function in favor of new device stack

This function is used for testing purposes only, there is no real use
for it in a user application. Remove in favor of implementation in new
device stack.
Also remove the part of the documentation that depends on loopback.
Documentation on how to implement your own USB device function using the
new USB device support will follow during the documentation rework.

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