Remove ConfigurationItem from API docs

`ConfigurationItem` appears in the API documentation for every service. See e.g. [SIMBAD](http://astroquery.readthedocs.org/en/latest/simbad.html#reference-api) or [Vizier](http://astroquery.readthedocs.org/en/latest/vizier.html#reference-api).

This is not useful and not the case in `astropy` or other affiliated packages.
What has to be changed so that it doesn't show up in API docs class list?