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

(cherry picked from commit 365b58ed)