Remove broken textContent functionality

[timlib]

Per prior issues, the textContent functionality does not produce valid text output. Issues have been closed as WONTFIX, so the best solution is to remove the functionality as it does not produce output which a typical user would rely on, and is therefore worse than not having it.

#779
#939

[gijsk]

Issues have been closed as WONTFIX

Except they haven't, #779 is still open? The other one I intended to close as a duplicate of 779, but for some reason github either didn't give me that option or did something else. I've fixed that now.

As noted in #779, I'd take a patch that tries to use innerText first, which would work with compliant DOM implementations. jsdom has an outstanding feature request for it, see jsdom/jsdom#1245 .

Let's try to keep the discussion in one place.