doc/conf.py: remove missing static files warning

Docs were configured to reference a static files directory, which
doesn't exist. Since we don't yet have any static files, I just
commented out this part of the configuration.

Prior to this change, you would see an error like this:

<pre>
$ cd doc && make html
sphinx-build -b html -d _build/doctrees   . _build/html
Running Sphinx v1.2.2
loading pickled environment... done
building [html]: targets for 4 source files that are out of date
updating environment: 0 added, 1 changed, 0 removed
reading sources... [100%] api                                                                                                                                                                             
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [100%] intro                                                                                                                                                                            
writing additional files... genindex py-modindex search
copying static files... WARNING: html_static_path entry u'/home/lars/proj/zpm/doc/_static' does not exist
done
copying extra files... done
dumping search index... done
dumping object inventory... done
build succeeded, 1 warning.

Build finished. The HTML pages are in _build/html.
</pre>