Open the Design tab's sketches only once their plane is picked

Sketch on a picked flat face or reference plane opens the sketch on it at once. With nothing
picked it no longer enters sketch mode: the reference planes and axes come up, and the plane or
flat face clicked next opens the sketch and puts them away. Esc or Cancel leaves without one.
A picked plane is used up by the sketch on it and dropped by Esc or a click on nothing, the plane
prompt is no longer replaced by a stale tool hint, and clicking the face a sketch was just
cancelled on picks that face again rather than the whole body.

src/slic3r/GUI/CAD/DesignCanvas.cpp

@@ -481,13 +481,6 @@ void DesignCanvas::begin_sketch(const SketchPlane& plane, DesignSketchTool::Mode
     if (m_canvas_widget) m_canvas_widget->Refresh();
 }
 
-void DesignCanvas::set_sketch_plane(const SketchPlane& plane)
-{
-    m_sketch_tool.set_plane(plane);   // keeps the 2D entities; only the carrier plane changes
-    if (m_canvas) m_canvas->set_as_dirty();
-    if (m_canvas_widget) m_canvas_widget->Refresh();
-}
-
 void DesignCanvas::edit_sketch(const std::vector<SketchEntity>& entities,
                                const std::vector<SketchEntityConstraintDef>& constraints,
                                const SketchPlane& plane)