webpack no need for ident if using complex options anymore (#2430)

Since `v2.2.1` of webpack the need for `ident` for complex options is no longer required. See

https://webpack.js.org/guides/migrating/#complex-options

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

@@ -223,7 +223,6 @@ module.exports = {
                 {
                   loader: require.resolve('postcss-loader'),
                   options: {
-                    ident: 'postcss', // https://webpack.js.org/guides/migrating/#complex-options
                     plugins: () => [
                       require('postcss-flexbugs-fixes'),
                       autoprefixer({