After setting memCacheHeight, .evict() is invalid

## code
```
CachedNetworkImage(
memCacheHeight: 200,
imageUrl:
'https://img0.baidu.com/it/u=3863723642,1799521238&fm=26&fmt=auto&gp=0.jpg');

ImageCache? imageCache = PaintingBinding.instance?.imageCache;
print(imageCache?.currentSize);
print(imageCache?.currentSizeBytes);
//-------------
CachedNetworkImageProvider(
"https://img0.baidu.com/it/u=3863723642,1799521238&fm=26&fmt=auto&gp=0.jpg",
).evict();
ImageCache? imageCache = PaintingBinding.instance?.imageCache;
print(imageCache?.currentSize);
print(imageCache?.currentSizeBytes);
```
## error
The picture still exists in memory

## version
cached_network_image: ^3.1.0
Flutter 2.2.3 • channel stable • https://github.com/flutter/flutter.git
Framework • revision f4abaa0735 (9 weeks ago) • 2021-07-01 12:46:11 -0700
Engine • revision 241c87ad80
Tools • Dart 2.13.4