Revert of [Sync] Refactor AttachmentStore classes. Introduce concept of referrer.

sync/api/attachments/attachment_store_backend.cc

Index: sync/api/attachments/attachment_store_backend.cc
diff --git a/sync/api/attachments/attachment_store_backend.cc b/sync/api/attachments/attachment_store_backend.cc
deleted file mode 100644
index 44363cc1a01a6087409f47e6f81aa48b680e4edd..0000000000000000000000000000000000000000
--- a/sync/api/attachments/attachment_store_backend.cc
+++ /dev/null
@@ -1,24 +0,0 @@
-// Copyright 2015 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/api/attachments/attachment_store_backend.h"
-
-#include "base/location.h"
-#include "base/sequenced_task_runner.h"
-
-namespace syncer {
-
-AttachmentStoreBackend::AttachmentStoreBackend(
-    const scoped_refptr<base::SequencedTaskRunner>& callback_task_runner)
-    : callback_task_runner_(callback_task_runner) {
-}
-
-AttachmentStoreBackend::~AttachmentStoreBackend() {
-}
-
-void AttachmentStoreBackend::PostCallback(const base::Closure& callback) {
-  callback_task_runner_->PostTask(FROM_HERE, callback);
-}
-
-}  // namespace syncer