Remove duplicated writeIndex func

Signed-off-by: Jin Dong <djdongjin95@gmail.com>

task.go

@@ -17,9 +17,7 @@
 package containerd
 
 import (
-	"bytes"
 	"context"
-	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
@@ -511,7 +509,7 @@ func (t *task) Checkpoint(ctx context.Context, opts ...CheckpointTaskOpts) (Imag
 			return nil, err
 		}
 	}
-	desc, err := t.writeIndex(ctx, &index)
+	desc, err := writeIndex(ctx, &index, t.client, t.id)
 	if err != nil {
 		return nil, err
 	}
@@ -662,18 +660,6 @@ func (t *task) checkpointImage(ctx context.Context, index *v1.Index, image strin
 	return nil
 }
 
-func (t *task) writeIndex(ctx context.Context, index *v1.Index) (d v1.Descriptor, err error) {
-	labels := map[string]string{}
-	for i, m := range index.Manifests {
-		labels[fmt.Sprintf("containerd.io/gc.ref.content.%d", i)] = m.Digest.String()
-	}
-	buf := bytes.NewBuffer(nil)
-	if err := json.NewEncoder(buf).Encode(index); err != nil {
-		return v1.Descriptor{}, err
-	}
-	return writeContent(ctx, t.client.ContentStore(), v1.MediaTypeImageIndex, t.id, buf, content.WithLabels(labels))
-}
-
 func writeContent(ctx context.Context, store content.Ingester, mediaType, ref string, r io.Reader, opts ...content.Opt) (d v1.Descriptor, err error) {
 	writer, err := store.Writer(ctx, content.WithRef(ref))
 	if err != nil {