tools: remove unused code in doc generation tool

tools/doc/html.js includes code that looks for comments with
`start-include` and `end-include` for file inclusion. This seems to be
legacy code that is no longer used. The code only appears in the
table-of-contents generation function. The strings `start-include` and
`end-include` appear nowhere else in our tools and nowhere at all in our
docs.

PR-URL: https://github.com/nodejs/node/pull/32913


Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>