Bug#1138817: Remove pointless caNv tags

> (The surrounding §5.7 "Defining chunks" also covers registering new public chunk types, so even non‑private new chunk types are an
> anticipated extension mechanism, not a violation.)

I didn't say “violation”. I wrote “nonstandard stuff”, which is fine in plain English; a more precise term would be “stuff void of semantics” or “semantics-less stuff”. You could also say “unknown private” and “ancillary” if you wish to stick to the terminology set by §5.4 of the PNG spec. I also wrote “pointless”. The caNv chunks in /usr/share/doc/texlive-doc/latex/webquiz/examples/*.png are like a biohazard in a bin tagged “don't open” - all goes well until someone, having his/her own logic, opens the bin. So if these caNv chunks serve some purpose - sure, state the purpose and go for it, but if they don't, let's simply get rid of the garbage. As of now, `convert /usr/share/doc/texlive-doc/latex/webquiz/examples/<whatever>.png /tmp/<whatever>.pdf` produce
 s a PDF with unexpectedly large margins.