Once you start developing your own app / lib, you should remove all unnecessary parts left from the sample app.

If you are sure your app will only be run in modern browser / node.js with Promise present, you can npm uninstall es6-promise --save, then remove es6-promise.amd.json file.

As the last step, make sure the dependency tree is in a correct state by recreating it: first delete your bower_components folder, then run bower install from your CLI. Similarly, delete your node_modules then run npm install from your CLI. Finally, rerun grunt devel.