function _closeRelatedDocument(liveDoc) {
        if (_relatedDocuments[liveDoc.doc.url]) {
            delete _relatedDocuments[liveDoc.doc.url];
        }

        if (_server) {
            _server.remove(liveDoc);
        }

        _closeDocument(liveDoc);
    }
Removes the given CSS/JSDocument from _relatedDocuments. Signals that the
given file is no longer associated with the HTML document that is live (e.g.
if the related file has been deleted on disk).