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/isosurf/isosurf.cpp

@@ -280,7 +280,7 @@ void TestGLCanvas::OnSize(wxSizeEvent& event)
     // This is OK here only because there is only one canvas that uses the
     // context. See the cube sample for that case that multiple canvases are
     // made current with one context.
-    const wxSize size = event.GetSize() * GetOpenGLScaleFactor();
+    const wxSize size = event.GetSize() * GetContentScaleFactor();
     glViewport(0, 0, size.x, size.y);
 }