[whatwg/xhr] Kill the progress event when readyState is 4 (#72)

Step 10 in [handle response end of file](https://xhr.spec.whatwg.org/#handle-response-end-of-file) isn't implemented by any web browser and the extra progress event seems somewhat superfluous since several other events can tell you when the request is done. I suggest removing step 10 to align the spec with current implementations.

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