Sql optimise admin get list of tags branch

## Testing Instructions:
In order to check the query execution log in to the admin console and click add new category. Then the query triggers.

![addnewcat](https://cloud.githubusercontent.com/assets/1329674/3902065/3db4850a-22b7-11e4-97b9-33754b08d439.png)

### Original Query before the Optimization

```sql
  SELECT a.id AS value, a.path, a.title AS text, a.level, a.published
  FROM ltzvy_tags AS a
  LEFT JOIN `ltzvy_tags` AS b 
  ON a.lft > b.lft 
  AND a.rgt  b.lft 
  AND a.rgt alias`  'root' 
  AND a.published IN (0,1)
  ORDER BY a.lft ASC
```

### Query Change Description
In this query `GROUP BY` clause has been removed and instead added the 'SELECT DISTINCT' clause in order to optimize the query execution
should be 
->select('DISTINCT a.id AS value, a.path, a.title AS text, a.level, a.published,a.lft')