[STUMP] Drop the generation of stumpwm.texi

To generate the info manual, StumpWM first processes `stumpwm.texi.in',
scrapping the lisp source code, producing `stumpwm.texi', so it later
can be compiled by `makeinfo'.

I think we can reduce the complexity of this process by removing the
scrapping phase and separate the manual from function docstrings, which
IMO have separate purposes.

I opened a pull request on github so we could have some concrete picture
if there is interest in discussing this.