qwidget_p.h: Remove DontSetCompositionMode from the internal flags

This flag was used by QWS to set or not set the composition mode
when filling the background. E.g. this was used by 'transparent'
windows to set the alpha bits. QPA didn't have this enabled in 4.8,
the code paths are gone in Qt5 and it is the responsibility of the
QPlatformBackingStore to initialize the alpha bits properly.

Change-Id: I4011d842b718d6a40b19922b5f75c597e63079c8
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>

src/widgets/kernel/qwidget_p.h

@@ -283,9 +283,8 @@ public:
         DrawRecursive = 0x04,
         DrawInvisible = 0x08,
         DontSubtractOpaqueChildren = 0x10,
-        DontSetCompositionMode = 0x20,
-        DontDrawOpaqueChildren = 0x40,
-        DontDrawNativeChildren = 0x80
+        DontDrawOpaqueChildren = 0x20,
+        DontDrawNativeChildren = 0x40
     };
 
     enum CloseMode {