Removing doubled description tab in template styles

Same as https://github.com/joomla/joomla-cms/pull/2481, but for template styles.

To test you can create a fieldset named "description" in the templateDetails.xml and add a field to it. The tab will show up twice.
Apply patch and it will show up only once.

Tracker: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=32957