Remove tranparent paint hack from BackdropFilter (flutter#29929)

We can remove the hack because flutter/engine#8183
has been rolled into the framework.

Author: liyuqian

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

@@ -1033,20 +1033,11 @@ class RenderBackdropFilter extends RenderProxyBox {
   @override
   bool get alwaysNeedsCompositing => child != null;
 
-  // TODO(liyuqian): remove this after updating the engine BackdropFilterLayer.
-  void _addTrasnparentPaint(PaintingContext context, Offset offset) {
-    // Draw a fully transparent paint to make sure that the cull rect won't be
-    // shrunk by Skia.
-    final Paint transparentPaint = Paint()..color = const Color(0x00000000);
-    context.canvas.drawPaint(transparentPaint);
-    super.paint(context, offset);
-  }
-
   @override
   void paint(PaintingContext context, Offset offset) {
     if (child != null) {
       assert(needsCompositing);
-      context.pushLayer(BackdropFilterLayer(filter: _filter), _addTrasnparentPaint, offset);
+      context.pushLayer(BackdropFilterLayer(filter: _filter), super.paint, offset);
     }
   }
 }