Fixed imgui out of synch with mouse after switching between preview and 3D view

src/slic3r/GUI/GLCanvas3D.hpp

@@ -425,10 +425,6 @@ private:
     Model* m_model;
     BackgroundSlicingProcess *m_process;
 
-#if ENABLE_SCROLLABLE_LEGEND
-    std::array<unsigned int, 2> m_old_size{ 0, 0 };
-#endif // ENABLE_SCROLLABLE_LEGEND
-
     // Screen is only refreshed from the OnIdle handler if it is dirty.
     bool m_dirty;
     bool m_initialized;