Remove now obsolete Shutdown method of ChromeBrowserStateManager.

ios/chrome/browser/application_context_impl.cc

Index: ios/chrome/browser/application_context_impl.cc
diff --git a/ios/chrome/browser/application_context_impl.cc b/ios/chrome/browser/application_context_impl.cc
index 1d56841c07ce2a4f241d5dae1a2e1777b7c97a92..eee2f06fa24e1e497af51d003213a1f2f39606f8 100644
--- a/ios/chrome/browser/application_context_impl.cc
+++ b/ios/chrome/browser/application_context_impl.cc
@@ -133,12 +133,7 @@ void ApplicationContextImpl::StartTearDown() {
   metrics_services_manager_.reset();
 
   // Need to clear browser states before the IO thread.
-  if (chrome_browser_state_manager_) {
-    // TODO(crbug.com/560854): the ShutDown() method can be folded into the
-    // destructor once ApplicationContextImpl owns ChromeBrowserStateManager.
-    chrome_browser_state_manager_->ShutDown();
-    chrome_browser_state_manager_.reset();
-  }
+  chrome_browser_state_manager_.reset();
 
   // PromoResourceService must be destroyed after the keyed services and before
   // the IO thread.