cros: Remove supervised user methods from SystemTrayDelegate

ash/system/supervised/custodian_info_tray_observer.h

-// 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 ASH_SYSTEM_SUPERVISED_CUSTODIAN_INFO_TRAY_OBSERVER_H_
-#define ASH_SYSTEM_SUPERVISED_CUSTODIAN_INFO_TRAY_OBSERVER_H_
-
-namespace ash {
-
-// Used to observe SystemTrayDelegate.
-class CustodianInfoTrayObserver {
- public:
-  // Called when information about the supervised user's custodian is changed,
-  // e.g. the display name.
-  virtual void OnCustodianInfoChanged() = 0;
-
- protected:
-  virtual ~CustodianInfoTrayObserver() {}
-};
-
-}  // namespace ash
-
-#endif  // ASH_SYSTEM_SUPERVISED_CUSTODIAN_INFO_TRAY_OBSERVER_H_