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

sync/internal_api/public/base/attachment_id_proto.h

Index: sync/internal_api/public/base/attachment_id_proto.h
diff --git a/sync/internal_api/public/base/attachment_id_proto.h b/sync/internal_api/public/base/attachment_id_proto.h
deleted file mode 100644
index 3332d8110366a88ee43c3612c180ca6e46be4cc2..0000000000000000000000000000000000000000
--- a/sync/internal_api/public/base/attachment_id_proto.h
+++ /dev/null
@@ -1,20 +0,0 @@
-// 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.
-
-#ifndef SYNC_INTERNAL_API_PUBLIC_BASE_ATTACHMENT_ID_PROTO_H_
-#define SYNC_INTERNAL_API_PUBLIC_BASE_ATTACHMENT_ID_PROTO_H_
-
-#include "sync/base/sync_export.h"
-#include "sync/protocol/sync.pb.h"
-
-namespace syncer {
-
-// Helper functions that are logically part of sync_pb::AttachmentIdProto.
-
-// Creates a unique AttachmentIdProto.
-SYNC_EXPORT_PRIVATE sync_pb::AttachmentIdProto CreateAttachmentIdProto();
-
-}  // namespace syncer
-
-#endif  // SYNC_INTERNAL_API_PUBLIC_BASE_ATTACHMENT_ID_PROTO_H_