CURRENT STATE: Once you create an Omnichannel (Advanced) queue and set it to default, you're stuck with it. You can't switch it back to not default, and thusly, you can't delete it. It's there forever now.

DESIRED STATE: You can switch an Omnichannel queue from default to not default, and you don't need to have another Omnichannel queue present as default, so that you can delete the queues as-needed.