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/penguin/penguin.cpp

@@ -306,7 +306,7 @@ void TestGLCanvas::ResetProjectionMode()
     // or more than one wxGLContext in the application.
     SetCurrent(*m_glRC);
 
-    const wxSize ClientSize = GetClientSize() * GetOpenGLScaleFactor();
+    const wxSize ClientSize = GetClientSize() * GetContentScaleFactor();
 
     // It's up to the application code to update the OpenGL viewport settings.
     // In order to avoid extensive context switching, consider doing this in