Background task for deleting existing non-local devices

[clokep]

Yes, I thought about an update task for deleting existing non-local devices, but I'm not sure how important it is compared to accidentally getting the query wrong and deleting everything in the table.

Originally posted by @uhoreg in #6956 (comment)

[clokep]

@richvdh had said

if we were to do such a clearout, we'd certainly want to fix #6870 first.

But that's now done, so I think that's fine?