Remove assert from Image._handleImageFrame() (#33602)

Tickers being disabled and re-enabled can cause the
condition of a synchronous notification happening after
image frames have been delivered, which is valid in that
case. As such, this removes the assert.

https://github.com/flutter/flutter/issues/32374

packages/flutter/lib/src/widgets/image.dart

@@ -936,7 +936,6 @@ class _ImageState extends State<Image> with WidgetsBindingObserver {
   }
 
   void _handleImageFrame(ImageInfo imageInfo, bool synchronousCall) {
-    assert(_frameNumber == null || !synchronousCall);
     setState(() {
       _imageInfo = imageInfo;
       _loadingProgress = null;