[iOS Clean] Removed old overlay implementation.

ios/clean/chrome/browser/ui/root/root_coordinator.mm

 // Copyright 2017 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.
 
 #import "ios/clean/chrome/browser/ui/root/root_coordinator.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
@@ skipping 38 matching lines @@
 }
 
 - (void)childCoordinatorDidStart:(BrowserCoordinator*)childCoordinator {
   self.viewController.contentViewController = childCoordinator.viewController;
 }
 
 - (void)childCoordinatorWillStop:(BrowserCoordinator*)childCoordinator {
   self.viewController.contentViewController = nil;
 }
 
-- (BOOL)canAddOverlayCoordinator:(BrowserCoordinator*)overlayCoordinator {
-  return YES;
-}
-
 #pragma mark - URLOpening
 
 - (void)openURL:(NSURL*)URL {
   [self.tabGridCoordinator openURL:URL];
 }
 
 @end