m.htmlsanity: drop Flash-related code and fix compat with docutils 0.17.

m.htmlsanity: drop Flash-related code and fix compat with docutils 0.17.

That thing is DEAD so there's no reason to bother. This changed in
docutils 0.17, making the current version fail there. Funnily enough,
https://github.com/live-clones/docutils/commit/659c3f1e46f0c86609936c987b7e2a7a6ef89cb2
looks like they still seem to preserve that somehow. I won't, so all the
related code isn't needed.

The code here was originally taken from docutils but diverged
significantly and given that they still use the deprecated width/height
attribute I see no reason to even attempt cherry-picking their changes.