Remove react-router as webpack external (remix-run#4487)

Author: pshrmn

packages/react-router-dom/webpack.config.js

@@ -12,12 +12,6 @@ module.exports = {
       commonjs2: 'react',
       commonjs: 'react',
       amd: 'react'
-    },
-    'react-router': {
-      root: 'ReactRouter',
-      commonjs2: 'react-router',
-      commonjs: 'react-router',
-      amd: 'react-router'
     }
   },