Allow tree models to be used in icon view

This removes an unnecessary check that prevents a single-layer TreeModel from being used in an icon view. Libxfce4ui shouldn't care what class model is being used; it just needs the model to be compatible, which only the application can ensure. (And even if the check were legitimate, it still shouldn't be done in a debug macro).

This check is currently preventing us from refactoring Thunar code (thunar#1361).