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

ios/chrome/browser/prefs/ios_chrome_pref_model_associator_client.cc

Index: ios/chrome/browser/prefs/ios_chrome_pref_model_associator_client.cc
diff --git a/ios/chrome/browser/prefs/ios_chrome_pref_model_associator_client.cc b/ios/chrome/browser/prefs/ios_chrome_pref_model_associator_client.cc
index 956e7f9c22ddacb72a8134319e32276805d19ab4..8642601a0db4e04c035c0c1af031f6ea6bb007e1 100644
--- a/ios/chrome/browser/prefs/ios_chrome_pref_model_associator_client.cc
+++ b/ios/chrome/browser/prefs/ios_chrome_pref_model_associator_client.cc
@@ -33,15 +33,3 @@ bool IOSChromePrefModelAssociatorClient::IsMergeableDictionaryPreference(
   }
   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;
-}