Remove docs generation from build

This is a "cardinal" solution to the issue, reported from Ubuntu packaging: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1009391

It seems, that there is a compatibility problem between new sphinx and old versions of importlib_metadata packages. One solution will be to force specific version of both of them, but then we will have to find working pairs for all pythons.

So after some discussion with @ajoubertza we decided, that we better remove the documentation generation from the setup.py at all, seems it is a bit outdated feature. No feature - no problems :-)