[whatwg/xhr] Drop the InvalidAccessError from the withCredentials setter when the synchronous flag is set (#66)

At this point only Gecko actually throws an exception, and a site broke:
https://bugzilla.mozilla.org/show_bug.cgi?id=1260515

Since the restriction is in a sense artificial and is unlikely to make much of an impact on overall sync XHR usage, dropping it seems OK.

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