Response image id but not digest when pull image

iSulad does not support using image name's format 'name@digest' to run
container currently, so response image id which iSulad support can
avoid failure caused by passing name@digest to iSulad by user in some
situation.
Signed-off-by: NWangFengTu <wangfengtu@huawei.com>

cmd/isulad_img/image_pull.go

@@ -117,10 +117,7 @@ func imagePull(gopts *globalOptions, popts *pullOptions, image string) (string,
 	if err != nil {
 		return "", err
 	}
-	imageRef := status.ID
-	if len(status.RepoDigests) > 0 {
-		imageRef = status.RepoDigests[0]
-	}
-	fmt.Print(imageRef)
-	return imageRef, nil
+
+	fmt.Print(status.ID)
+	return status.ID, nil
 }