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.
yeah, ``~QStandardItemModel`` will be called during destruction. but the patch ensures ``virtual_repos_catetory_`` is destructed after all its items destroyed.

In a normal destruction of ``RepoTreeModel``, ``~QStandardItemModel`` will be called at L54 ``}``, just right after the lines of the body are executed.