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 Ref: https://github.com/jerrykuku/luci-theme-argon/pull/422