Remove year+ old migration support of "session.urls_to_restore_on_startup".

ios/chrome/browser/prefs/ios_chrome_pref_model_associator_client.cc

 // Copyright 2015 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.
 
 #include "ios/chrome/browser/prefs/ios_chrome_pref_model_associator_client.h"
 
 #include "base/memory/singleton.h"
 #include "components/content_settings/core/browser/website_settings_info.h"
 #include "components/content_settings/core/browser/website_settings_registry.h"
 
@@ skipping 15 matching lines @@
 bool IOSChromePrefModelAssociatorClient::IsMergeableDictionaryPreference(
     const std::string& pref_name) const {
   const content_settings::WebsiteSettingsRegistry& registry =
       *content_settings::WebsiteSettingsRegistry::GetInstance();
   for (const content_settings::WebsiteSettingsInfo* info : registry) {
     if (info->pref_name() == pref_name)
       return true;
   }
   return false;
 }
-
-bool IOSChromePrefModelAssociatorClient::IsMigratedPreference(
-    const std::string& new_pref_name,
-    std::string* old_pref_name) const {
-  return false;
-}
-
-bool IOSChromePrefModelAssociatorClient::IsOldMigratedPreference(
-    const std::string& old_pref_name,
-    std::string* new_pref_name) const {
-  return false;
-}