Actually, you can delete photos saved by your app (saved to photo library with UIImageWriteToSavedPhotosAlbum API call).
The documented API [ALAsset setImageData:metadata:completionBlock:] works.
1). Add an image "photo.jpg" to your project 
2). Save an image to asset library:
ALAssetsLibrary *lib = [ALAssetsLibrary new];
UIImage *image = [UIImage imageNamed:@"photo.jpg"];
[lib writeImageToSavedPhotosAlbum:image.CGImage metadata:@{} completionBlock:^(NSURL *assetURL, NSError *error) {
    NSLog(@"Write image %@ to asset library. (Error %@)", assetURL, error);
}];
3). Go to default gallery, you will find photo.jpg in your "Saved Photos" album.
4). Delete this image from asset library:
ALAssetsLibrary *lib = [ALAssetsLibrary new];
[lib enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
    [group enumerateAssetsUsingBlock:^(ALAsset *asset, NSUInteger index, BOOL *stop) {
        if(asset.isEditable) {
            [asset setImageData:nil metadata:nil completionBlock:^(NSURL *assetURL, NSError *error) {
                NSLog(@"Asset url %@ should be deleted. (Error %@)", assetURL, error);
            }];
        }
    }];
} failureBlock:^(NSError *error) {
}];
5). Go to default gallery, you will find photo.jpg has already been deleted.