removed techniques should not be output

[michael-n-cooper]

https://www.w3.org/WAI/WCAG22/Techniques/html/H4 is a technique that isn't referenced an Understanding document (most likely removed), but still is output and published. We might want to remove the technique, or swap it for a "no longer used" page.

[fstrr]

I've deleted H4 for that reason in the increasingly large Techniques-related PR. Out of interest, do we have a "no longer used" page—Alastair and I were talking recently about if such a thing existed and, if it did, going through the Techniques TOC and linking references to deleted Techniques to it.