Remove debugDoingPaint setter.

debugDoingPaint is an internal flag and really shouldn't be globally settable.

packages/flutter/lib/src/rendering/object.dart

@@ -818,9 +818,6 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
 
   static bool _debugDoingPaint = false;
   static bool get debugDoingPaint => _debugDoingPaint;
-  static void set debugDoingPaint(bool value) {
-    _debugDoingPaint = value;
-  }
   bool _debugDoingThisPaint = false;
   bool get debugDoingThisPaint => _debugDoingThisPaint;
   static RenderObject _debugActivePaint = null;