[Extensions] Remove principalsPrivate API

chrome/common/extensions/api/principals_private.idl

-// Copyright 2013 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.
-
-// Experimental APIs to trigger Chrome sign in actions.
-// Only enabled if the flag "new-profile-management" is set.
-namespace principalsPrivate {
-
-  interface Functions {
-    // Triggers Chrome sign out.
-    // Only enabled if the flag "new-profile-management" is set.
-    static void signOut();
-
-    // Shows the avatar bubble.
-    // Only enabled if the flag "new-profile-management" is set.
-    static void showAvatarBubble();
-  };
-};