fix(postcss): do not output custom properties in resulting css

Because this creates a lot of duplication when importing css variables
files in other css files which are then imported via js.
See:
- postcss/postcss-import#316
- postcss/postcss-import#294
- postcss/postcss-import#224

packages/postcss/mixin.core.js

@@ -36,6 +36,9 @@ const getCSSLoaderConfig = browsers => ({
             features: {
               'nesting-rules': true,
               'custom-media-queries': true,
+              'custom-properties': {
+                preserve: false,
+              },
             },
           }),
         ],