removed options in UglifyJsPlugin which are equal to the defaults (#1993

)

* removed redundant already default options in UglifyJsPlugin

* re-enabled remove comments Uglify option

JSDoc-style comments weren't being removed

packages/react-scripts/config/webpack.config.prod.js

@@ -258,15 +258,10 @@ module.exports = {
     // Minify the code.
     new webpack.optimize.UglifyJsPlugin({
       compress: {
-        screw_ie8: true, // React doesn't support IE8
         warnings: false,
       },
-      mangle: {
-        screw_ie8: true,
-      },
       output: {
         comments: false,
-        screw_ie8: true,
       },
       sourceMap: true,
     }),