Remove the EPUB output.

Rationale:
- The Sphinx EPUB builder is broken in at least two ways (mime type
  issue and search index issue). While the sources of these problems
  have been identified, worked around in our Waf-Sphinx tool, and could
  be fixed in Sphinx, EPUB does not in general seem to be well-supported
  in Sphinx.
- Our EPUB output at present looks pretty bad. We have not been checking
  it with diligence, and the format is not well-suited to technical
  documentation with frequent tables and code blocks.

The EPUB output is cool but nothing else. It's not useful, professional, practical, or time-efficient.