css: remove postcss-flexbugs-fixes plugin.

Remove a PostCSS plugin used to workaround bugs in the `flex` CSS
property. Most of these bugs are no longer relevant and only affected
ancient browsers like IE 10 or 11.

https://github.com/philipwalton/flexbugs

postcss.config.js

@@ -1,7 +1,6 @@
 module.exports = {
   plugins: [
     require('postcss-import'),
-    require('postcss-flexbugs-fixes'),
     require('postcss-preset-env')({
       autoprefixer: {
         flexbox: 'no-2009'