remove hardcoded binary paths once supervisord $PATH is extended

U7#714 should be fixed soon, adding the `/opt/uberspace/etc/isabell/binpaths/ruby` magic to `$PATH`, as well as other ones like `~/.local/bin`:
* only `puma` is needed in [the puma guide](https://github.com/Uberspace/lab/blob/master/source/guide_puma.rst).
* only `uwsgi` is needed in the `includes/install-uwsgi.rst`