One more attempt to cleanup librustdoc used by bookrunner (#1191)

* Delete render folder

It builds but it's completely untested.

* Removed a bunch of unused functions

* Remove more deadcode.

I also removed the tests since they were interfering with the usage
analysis.

* Remove dead modules

* More unused functions and modules

* Remove collect_intra_doc_links + new deadcode

* Remove unindent_comments + new unused imports

I don't see how bookrunner would be influenced by the number of spaces
in the code comment. Remove it!

* Cleanup markdown.rs + new deadcode

* Remove readonly fields

Author: celinval