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.

include/wx/gtk/glcanvas.h

@@ -60,8 +60,6 @@ public:
 
     virtual bool SetBackgroundStyle(wxBackgroundStyle style) wxOVERRIDE;
 
-    virtual double GetOpenGLScaleFactor() const wxOVERRIDE;
-
     // implement wxGLCanvasX11 methods
     // --------------------------------