rm doc-generation code that made mostly-unused docs

This code worked hard to generate some documentation pages.
The Google analytics numbers point out: these pages were largely unread.
Some (but not all) of this code broke in a refactor: it generated empty docs.
Yet nobody complained.

Rather than work hard to fix and maintain this code, I'm ripping it out.
Behold the fiery debate: https://groups.google.com/forum/#!topic/pants-devel/f7V4_6kLdZs

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/30318159

Reviewed at https://rbcommons.com/s/twitter/r/704/