Revert of Keep track of which attachments are referenced by which sync entries.

sync/internal_api/public/base/attachment_id_proto.cc

-// Copyright 2014 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#include "sync/internal_api/public/base/attachment_id_proto.h"
-
-#include "base/rand_util.h"
-
-namespace syncer {
-
-sync_pb::AttachmentIdProto CreateAttachmentIdProto() {
-  // Only requirement here is that this id must be globally unique.
-  // TODO(maniscalco): Consider making this base64 encoded.
-  sync_pb::AttachmentIdProto proto;
-  proto.set_unique_id(base::RandBytesAsString(16));
-  return proto;
-}
-
-}  // namespace syncer