[xhr] first step of open() method does not work in workers anymore (#47)

The setup in HTML changed and as @Ms2ger pointed out the first step of `open()` now no longer makes sense.

I think that workers do not run if they're not in the active document so maybe we don't need that step to apply to workers at all...

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/xhr/issues/47