remove GLModel from base class

- only 2 gizmo use cone

(cherry picked from commit prusa3d/PrusaSlicer@3c7390e34e)
(cherry picked from commit 4647fc49ef5b925432e087252f32a6601d0410da)

src/slic3r/GUI/Gizmos/GLGizmoRotate.cpp

@@ -76,6 +76,7 @@ std::string GLGizmoRotate::get_tooltip() const
 
 bool GLGizmoRotate::on_init()
 {
+    m_cone.init_from(its_make_cone(1., 1., 2 * PI / 24));
     m_grabbers.push_back(Grabber());
     return true;
 }