wallpapers/potd: remove unsplash provider
https://source.unsplash.com has been deprecated and doesn't work anymore. To use Unsplash one needs a developer access key and use https://api.unsplash.com
The API Guidelines say:
You cannot replicate the core user experience of Unsplash (unofficial clients, wallpaper applications, etc.).
a wallpaper app returns Unsplash images for downloading. Without the integration, the app has no content and no value to users.
The API is to be used for non-automated, high-quality, and authentic experiences.
So the Unsplash provider has to be removed.
BUG: 471526 FIXED-IN: 5.27.7
Edited by Fushan Wen