PartDesign: Primitives are attachable directly

With the recently introduced AttachExtension the PartDesign primitives can be attachable by themself and don't need anymore the helping coordinate system datum. This simplyfies the code and the TreeView largely

src/Mod/PartDesign/Gui/ViewProviderPrimitive.h

@@ -40,7 +40,6 @@ public:
     /// destructor
     virtual ~ViewProviderPrimitive();
     
-    virtual std::vector< App::DocumentObject* > claimChildren(void) const;
     virtual void attach(App::DocumentObject*);
     virtual void updateData(const App::Property*);