Remove polyfill libraries

[danielghost]

Now that we no longer need to support earlier browsers which don't support ES6, investigate whether the polyfills for URL, Fetch and Promises can be removed.

The Fetch API may be the only one which may require further investigation as the keepalive: true setting is not supported by Firefox, so we need to be mindful of data loss as transmission occurs when closing the browser. The Beacon API which is supported by Firefox didn't allow custom headers as required for xAPI when this was prevously looked at years ago. Need to check if there is still the case or if there is a new alternative.