Fixed issue with description tab showing twice

http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=32635

If you have a fieldset with name 'description', it will show twice.
This is the case for modules and plugins.

This PR fixes that issue.

#### Testing instructions
Install a plugin and module that use the 'description' fieldset and check the plugin/module settings of that extension.
To test you could use this extension:
http://www.nonumber.nl/extensions/cachecleaner
This extension comes with an admin module and a system plugin.