Remove wxHAS_BITMAP_SCALE_FACTOR

Now wxMSW also stores the scale factor, even if it doesn't use it in its
GetScaledXXX(), so it doesn't seem useful to have this symbol for
distinguishing the platforms with and without bitmap scale factor
support, when we can just use wxHAS_DPI_INDEPENDENT_PIXELS instead in
the only place where this was used.

And as this symbol was added quite recently, in 2c1f4c002d (Add
wxBitmap::SetScaleFactor(), 2021-10-23), we can hopefully just remove it
without breaking any existing code, if we do it right now.

include/wx/gtk/bitmap.h

@@ -17,10 +17,6 @@ typedef struct _GdkPixbuf GdkPixbuf;
 class WXDLLIMPEXP_FWD_CORE wxPixelDataBase;
 class WXDLLIMPEXP_FWD_CORE wxCursor;
 
-#ifdef __WXGTK3__
-    #define wxHAS_BITMAP_SCALE_FACTOR
-#endif
-
 //-----------------------------------------------------------------------------
 // wxMask
 //-----------------------------------------------------------------------------
@@ -89,7 +85,7 @@ public:
         { return Create(sz.GetWidth(), sz.GetHeight(), depth); }
     bool Create(int width, int height, const wxDC& WXUNUSED(dc))
         { return Create(width,height); }
-#ifdef wxHAS_BITMAP_SCALE_FACTOR
+#ifdef __WXGTK3__
     virtual bool CreateScaled(int w, int h, int depth, double scale) wxOVERRIDE;
     virtual void SetScaleFactor(double scale) wxOVERRIDE;
     virtual double GetScaleFactor() const wxOVERRIDE;