Removed CopyState() from NavigationManager, as it is unused.

ios/web/navigation/navigation_manager_impl.mm

Index: ios/web/navigation/navigation_manager_impl.mm
diff --git a/ios/web/navigation/navigation_manager_impl.mm b/ios/web/navigation/navigation_manager_impl.mm
index 539998a100e00a180f6c513fd16974314eef6e36..76c60465eed16d8f95eb060d4ad4700a747af76b 100644
--- a/ios/web/navigation/navigation_manager_impl.mm
+++ b/ios/web/navigation/navigation_manager_impl.mm
@@ -359,11 +359,6 @@ bool AreURLsInPageNavigation(const GURL& existing_url, const GURL& new_url) {
   transient_url_rewriters_.reset();
 }
 
-void NavigationManagerImpl::CopyState(
-    NavigationManagerImpl* navigation_manager) {
-  SetSessionController([navigation_manager->GetSessionController() copy]);
-}
-
 int NavigationManagerImpl::GetIndexForOffset(int offset) const {
   int result = [session_controller_ pendingItemIndex] == -1
                    ? GetCurrentItemIndex()