Fix deleting in JHelperTags for #_ucm_content only entries

Hi,
Currently JHelperTags not considering about purely UCM data when deleting last tag. It's just remove entry from #_ucm_content table when saving after removing last tag.
This fix checks the whether $table type is JTableCorecontent and if so, not remove #_ucm_content entry.

 Issue Tracker Item: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=34036&start=0
Can we remove the else here? It's just nicer code style wise to me personally. So

```
if ($table instanceof JTableCorecontent)
{
    return $result;
}

return $result && $ucmContentTable->deleteByContentId($contentItemId, $this->typeAlias);