prevent memory leak from RepoTreeModel

object ``virtual_repos_catetory_`` is created without being used. 
this patch simply comments this line out to prevent memory leak.
I don't think we need to call `clear()` here, because the base class `QStandardItemModel` would do that in its dtor.

```
QStandardItemModel::~QStandardItemModel ()
Destructs the model. The model destroys all its items.
```