Remove redundant drag'n'drop interface

In the long time of parallel assembly development an own drag'n'drop interface has been added to master and assembly branch. After merge both have been available. This commit removes one and keeps the more extensive implementation

src/Gui/ViewProviderDocumentObjectGroup.h

@@ -47,16 +47,6 @@ public:
     std::vector<std::string> getDisplayModes(void) const;
     bool isShow(void) const;
 
-    /// get called if the user hover over a object in the tree
-    virtual bool allowDrop(const std::vector<const App::DocumentObject*> &objList,
-                           Qt::KeyboardModifiers keys,
-                           Qt::MouseButtons mouseBts,
-                           const QPoint &pos);
-    /// get called if the user drops some objects
-    virtual void drop(const std::vector<const App::DocumentObject*> &objList,
-                      Qt::KeyboardModifiers keys,
-                      Qt::MouseButtons mouseBts,
-                      const QPoint &pos);
 
 protected:
     void getViewProviders(std::vector<ViewProviderDocumentObject*>&) const;