[CSS] Unbind css.liquid extension (#3440)

Proper syntax highlighting of CSS files with liquid interpolation is
provided by https://github.com/SublimeText/Liquid

Using default CSS syntax for such files may cause highlighting issues
as soon as Liquid style interpolation is used.

Example:

```
a {
   font-{{family}}: "{{font}}";
}
```

CSS/CSS.sublime-syntax

@@ -8,7 +8,6 @@ version: 2
 
 file_extensions:
   - css
-  - css.liquid
 
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