Remove support for PushSubscription.subscriptionId.

Source/modules/push_messaging/PushSubscription.idl

 // 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.
 
 // https://w3c.github.io/push-api/#pushsubscription-interface
 
 [
     Exposed=(Window,ServiceWorker),
     GarbageCollected,
     RuntimeEnabled=PushMessaging,
 ] interface PushSubscription {
     readonly attribute USVString endpoint;
-    [DeprecateAs=PushSubscriptionId] readonly attribute DOMString subscriptionId;
     [CallWith=ScriptState] Promise<boolean> unsubscribe();
 
     serializer = { attribute };
 };