[Sync] Remove backend unrecoverable error handler

chrome/browser/sync/backend_unrecoverable_error_handler.h

-// Copyright (c) 2012 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 CHROME_BROWSER_SYNC_BACKEND_UNRECOVERABLE_ERROR_HANDLER_H_
-#define CHROME_BROWSER_SYNC_BACKEND_UNRECOVERABLE_ERROR_HANDLER_H_
-
-#include <string>
-
-#include "base/location.h"
-#include "base/memory/weak_ptr.h"
-
-#include "sync/internal_api/public/util/unrecoverable_error_handler.h"
-#include "sync/internal_api/public/util/weak_handle.h"
-
-class ProfileSyncService;
-namespace browser_sync {
-
-class BackendUnrecoverableErrorHandler
-    : public syncer::UnrecoverableErrorHandler {
- public:
-  BackendUnrecoverableErrorHandler(
-      const syncer::WeakHandle<ProfileSyncService>& service);
-  ~BackendUnrecoverableErrorHandler() override;
-  void OnUnrecoverableError(const tracked_objects::Location& from_here,
-                            const std::string& message) override;
-
- private:
-  syncer::WeakHandle<ProfileSyncService> service_;
-};
-}  // namespace browser_sync
-#endif  // CHROME_BROWSER_SYNC_BACKEND_UNRECOVERABLE_ERROR_HANDLER_H_
-