Ref T472, heap issue follow up + minor clean ups

* unfortunately the issue did still persist
* reason still unknown
* not expanding all seems to solve it

Maybe using the high level API without own model would be better
https://www.bogotobogo.com/Qt/Qt5_QTreeWidget.php

src/blackgui/views/aircraftcategorytreeview.h

@@ -69,9 +69,6 @@ namespace BlackGui
             //! The selected object
             BlackMisc::Aviation::CAircraftCategory selectedObject() const;
 
-            //! Suffix for index
-            QString suffixForIndex(const QModelIndex &index);
-
             //! Expanded
             void onExpanded(const QModelIndex &index);