ArcGIS Server has an endpoint for unregistering a replica, and the Runtime SDK has an API for unregistering a geodatabase that hits this endpoint.
But here's my challenge..
Say a user wants to delete their offline data. Should you unregister the geodatabase? What if you don't? If you're online, then you can easily unregister and delete. But if you're offline, you can delete, but you can't immediately unregister. Not only that, but the API requires you to actually have a .geodatabase file in order to call unregister. So because you need to have a .geodatabase to unregister, and because you can't unregister when offline, it means that you cannot delete offline.
The only workaround I can see for this is to delete the file when offline, then record an indication somewhere that this geodatabase needs to be unregistered when you come back online, and then do the unregistration when online. But since the Runtime API is not capable of unregistering without a .geodatabase file, I will need to write the HTTP request logic to perform the unregister myself.