Refactor: remove wxNotebook::DoDrawBackground() in wxMSW

The code in QueryBgBitmap() and MSWPrintChild() is sufficiently different that
we can't easily reuse the drawing calls between them, so don't tie ourselves
in knots trying to do it, just duplicating these 2 calls in the 2 functions is
not that bad and the code is more clear.

No real changes.

include/wx/msw/notebook.h

@@ -178,11 +178,6 @@ protected:
 
   // creates the brush to be used for drawing the tab control background
   void UpdateBgBrush();
-
-  // common part of QueryBgBitmap() and MSWPrintChild()
-  //
-  // if child == NULL, draw background for the entire notebook itself
-  bool DoDrawBackground(WXHDC hDC, wxWindow *child = NULL);
 #endif // wxUSE_UXTHEME
 
   // these function are used for reducing flicker on notebook resize