remove GLModel from base class

- only 2 gizmo use cone

src/slic3r/GUI/Gizmos/GLGizmoRotate.cpp

@@ -91,6 +91,7 @@ void GLGizmoRotate::disable_grabber() { m_grabbers[0].enabled = false; }
 
 bool GLGizmoRotate::on_init()
 {
+    m_cone.init_from(its_make_cone(1., 1., 2 * PI / 24));
     m_grabbers.push_back(Grabber());
     return true;
 }