Drop IE 11 support by default (#5090)

* Drop ie 11 support and move polyfills to a new package

* More useful directions for what entry point to use
https://github.com/facebook/create-react-app/pull/5090#discussion_r220313783

* Clear up what file this polyfill goes in
https://github.com/facebook/create-react-app/pull/5090#discussion_r220313980

* Polyfill `window`, not `global`

* Remove proxy polyfill file

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

@@ -100,8 +100,8 @@ module.exports = {
   // We generate sourcemaps in production. This is slow but gives good results.
   // You can exclude the *.map files from the build during deployment.
   devtool: shouldUseSourceMap ? 'source-map' : false,
-  // In production, we only want to load the polyfills and the app code.
-  entry: [require.resolve('./polyfills'), paths.appIndexJs],
+  // In production, we only want to load the app code.
+  entry: [paths.appIndexJs],
   output: {
     // The build folder.
     path: paths.appBuild,