Remove ignored postcss options

Info about ignoring the postcss option when the postcss.config.js file is present.
https://nuxtjs.org/api/configuration-build#postcss

examples/tailwindcss/nuxt.config.js

@@ -1,9 +1,3 @@
 module.exports = {
-  build: {
-    postcss: [
-      require('tailwindcss')('./tailwind.js'),
-      require('autoprefixer')
-    ]
-  },
   css: ['~/assets/css/tailwind.css']
 }