Support for image style flush on S3

Problem

When I tried to flush the image style cache via UI (or drush) with S3 as $scheme, the styles are not getting flushed.

ImageStyle::flush() does below to delete the files/directories.

      if (file_exists($derivative_uri)) {
        file_unmanaged_delete($derivative_uri);
      }

I believe these functions has no effect with S3.