Revert "perf(netlify-cms-core): add basic route code-splitting (#1889)"

This reverts commit 9aa56457e297a9e2592ca87a59b635508ef5702e.

packages/netlify-cms-core/package.json

@@ -39,10 +39,10 @@
     "netlify-cms-ui-default": "^2.3.0",
     "node-polyglot": "^2.3.0",
     "prop-types": "^15.5.10",
-    "react": "^16.6.3",
+    "react": "^16.4.1",
     "react-dnd": "^7.0.0",
     "react-dnd-html5-backend": "^7.0.0",
-    "react-dom": "^16.6.3",
+    "react-dom": "^16.0.0",
     "react-emotion": "^9.2.5",
     "react-frame-component": "^4.0.2",
     "react-hot-loader": "^4.0.0",