CSS-preprocessor: remove gcss (#5682)

- last change is 11 years old
- does not provide go module at all
- yosssi/gcss#21

Author: gucio321

README.md

@@ -538,7 +538,6 @@ _Tools for help with continuous integration._
 
 _Libraries for preprocessing CSS files._
 
-- [gcss](https://github.com/yosssi/gcss) - Pure Go CSS Preprocessor.
 - [go-libsass](https://github.com/wellington/go-libsass) - Go wrapper to the 100% Sass compatible libsass project.
 
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