npm install laravel-mix-purgecss --save-dev
  
const mix = require('laravel-mix');
require('laravel-mix-purgecss');

// ...

mix.js('resources/js/app.js', 'public/js')
   .sass('resources/sass/app.scss', 'public/css')
   .purgeCss(); //Add this line and that'll do the job