Remove Universal Sky as author removed their GitHub account

A GitHub search reveals there is one fork that was turned into a
source repository:

https://github.com/dwlcj/UniversalSky

However, it's not maintained anymore.

README.md

@@ -239,7 +239,6 @@ See [Vivraan/godot-lang-support](https://github.com/Vivraan/godot-lang-support).
 - [SmartShape2D](https://github.com/SirRamEsq/SmartShape2D) - A 2D terrain tool.
 - [Tree generator](https://github.com/Zylann/godot_tree_generator_plugin) - Tool for generating trees.
 - [UI Design Tool](https://github.com/imjp94/UIDesignTool) - Intuitive design workflow for UIs in the editor.
-- [Universal Sky](https://github.com/7leodev/UniversalSky) - Adds a Dynamic Sky and Time of Day.
 - [Volumetrics](https://github.com/SIsilicon/Godot-Volumetrics-Plugin) - Voxel-based volumetric lighting and fog plugin.
 - [Voxel-Core](https://github.com/ClarkThyLord/Voxel-Core) - GDScript voxel plugin for creating, importing and editing voxel content in-engine and in-game.
 - [VPainter](https://github.com/tomankirilov/VPainter) - 3D vertex painting plugin.