Update webpack prod config

webpack.client.prod.js

 const webpack = require('webpack');
 const merge = require('webpack-merge');
-const UglifyJSPlugin = require('uglifyjs-webpack-plugin');
 const common = require('./webpack.client.common.js');
 
 module.exports = merge(common, {
   mode: 'production',
   devtool: 'source-map',
   plugins: [
-    new UglifyJSPlugin({
-      sourceMap: true
-    }),
     new webpack.DefinePlugin({
       'process.env.NODE_ENV': JSON.stringify('production')
     })