POWER: Combining document and settings object checks.

Jeff noted in https://github.com/w3c/webappsec/issues/160 that it was more
than confusing to have two checks in POWER, one for Documents, and one for
settings objects. Since we can get to the one from the other, we should
have one check to avoid boilerplate in every other spec.

I've combined the checks into
https://w3c.github.io/webappsec/specs/powerfulfeatures/#settings-sufficiently-secure.
I'd appreciate feedback regarding the new text's sanity. :)

Thanks!

--
Mike West <mkwst@google.com>, @mikewest

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