Delete debug printf from commit 2e7c6a3
More updates to xexprs->scribble-pres

- The markdown parser generates xexprs that always have attributes,
  including () for no attributes.

- Tables don't necessarily have a tbody

- Images are now in a div with caption.

- Plain code blocks are always `(pre () (code () ___))`.

This is still not 100% correct:

- See commented-out unit test for table -> tabular.

- Footnotes not yet handled.