Remove recently added GetOpenGLScaleFactor()

It has become unnecessary after the previous commit, as now the generic
GetContentScaleFactor() can be used instead of it on all platforms, so
revert the changes of f6cc8ff52c (Add GetOpenGLScaleFactor() to abstract
OpenGL coordinates scaling, 2020-07-10).

See https://github.com/wxWidgets/wxWidgets/pull/1944

See #17391.

samples/opengl/cube/cube.cpp

@@ -349,7 +349,7 @@ void TestGLCanvas::OnPaint(wxPaintEvent& WXUNUSED(event))
     // multiple canvases: If we updated the viewport in the wxSizeEvent
     // handler, changing the size of one canvas causes a viewport setting that
     // is wrong when next another canvas is repainted.
-    const wxSize ClientSize = GetClientSize() * GetOpenGLScaleFactor();
+    const wxSize ClientSize = GetClientSize() * GetContentScaleFactor();
 
     TestGLContext& canvas = wxGetApp().GetContext(this, m_useStereo);
     glViewport(0, 0, ClientSize.x, ClientSize.y);