Remove device source specific Ubidots label generation

When the TTN ubidots function is finally switched off after the broker can handle all traffic, remove the source specific Ubidots label generation code.

We cannot do this as long as the ubifunction is handling any traffic and might create a new Ubidots device, because the broker must be able to arrive at the same device label as the ubifunction so duplicate devices are not created.

After the ubifunction is switched off we can use random UUIDs for device labels.