css: don't include fallbacks for CSS variables.

Don't include fallback CSS properties for CSS variables. Before we
generated CSS like this:

  a.tag-type-1 {
    color: #c00004;
    color: var(--artist-tag-color);
  }

Now we generate CSS like this:

  a.tag-type-1 {
    color: var(--artist-tag-color);
  }

This means that support for CSS variables is now required for colors to
work properly. All major browsers have supported CSS variables since
2016-2017.

postcss.config.js

@@ -6,10 +6,6 @@ module.exports = {
       autoprefixer: {
         flexbox: 'no-2009'
       },
-      // https://github.com/csstools/postcss-preset-env#importfrom
-      importFrom: [
-        'app/javascript/src/styles/base/040_colors.css'
-      ],
       stage: 3
     })
   ]