fix: tree-shaking (#2881)

- Removed deprecated `jsnext:main`

Author: Can-Sahin

internals/webpack/webpack.base.babel.js

@@ -116,7 +116,6 @@ module.exports = options => ({
   resolve: {
     modules: ['node_modules', 'app'],
     extensions: ['.js', '.jsx', '.react.js'],
-    mainFields: ['browser', 'jsnext:main', 'main'],
     alias: {
       'react-dom': '@hot-loader/react-dom',
     },