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/glcanvas.h

@@ -258,11 +258,6 @@ public:
     // as a parameter
     wxGLContextAttrs& GetGLCTXAttrs() { return m_GLCTXAttrs; }
 
-    // Return the factor to apply to transform window coordinates (e.g. sizes)
-    // to OpenGL coordinates: it can be different from 1 on the platforms where
-    // logical pixels are different from physical ones (i.e. wxGTK3 and wxOSX).
-    virtual double GetOpenGLScaleFactor() const;
-
     // deprecated methods using the implicit wxGLContext
 #if WXWIN_COMPATIBILITY_2_8
     wxDEPRECATED( wxGLContext* GetContext() const );