Parts of canvas tests relating to generating an annotated spec should be removed

[adroitwhiz]

Previously, running the canvas tests' build.sh generated an annotated version of the HTML canvas spec.

This functionality has been removed in dadd085, but some cruft has been left behind:

• html/canvas/tools/current-work-canvas.xhtml, which is a stripped-down version of the canvas spec previously used to generate the annotated version.
• html/canvas/tools/specextract.py, which generates current-work-canvas.xhtml.
• Weird syntax in html/canvas/tools/spec.yaml (e.g. <^>, <...>, <eol>), which, as far as I can tell, only exists to aid in the generation of the annotated spec.
• The previously field, also in spec.yaml, which appears to be unused now.
• The htmlSerializer and getNodeText functions in html/canvas/tools/gentestutils.py, which was used only to serialize the annotated spec.

These should be removed if possible--I'm trying to write tests for a proposal that modifies a substantial amount of the canvas spec, and having so many supporting changes in place for removed functionality led me down a rabbit hole of confusion.