Clean up usage of "OnNavigationItemsPruned"

WebStateObserver::NavigationItemsPruned is part of the legacy navigation
and is already cleaned up in https://crrev.com/c/1953765. This CL
cleans up remaining usage in //ios.

Bug: 1029306
Change-Id: I5e96c868f44100f28a13b7cba2ba797b8c938644
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1960449Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Yi Su <mrsuyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#723493}

ios/chrome/browser/ui/location_bar/location_bar_mediator.mm

@@ -112,11 +112,6 @@
   [self notifyConsumerOfChangedSecurityIcon];
 }
 
-- (void)webState:(web::WebState*)webState
-    didPruneNavigationItemsWithCount:(size_t)pruned_item_count {
-  DCHECK_EQ(_webState, webState);
-}
-
 - (void)webStateDidStartLoading:(web::WebState*)webState {
   DCHECK_EQ(_webState, webState);
   [self notifyConsumerOfChangedLocation];

ios/chrome/browser/ui/popup_menu/popup_menu_mediator.mm

@@ -219,12 +219,6 @@ PopupMenuToolsItem* CreateTableViewItem(int titleID,
   [self updatePopupMenu];
 }
 
-- (void)webState:(web::WebState*)webState
-    didPruneNavigationItemsWithCount:(size_t)pruned_item_count {
-  DCHECK_EQ(_webState, webState);
-  [self updatePopupMenu];
-}
-
 - (void)webStateDidStartLoading:(web::WebState*)webState {
   DCHECK_EQ(_webState, webState);
   [self updatePopupMenu];

ios/chrome/browser/ui/toolbar/toolbar_mediator.mm

@@ -114,12 +114,6 @@
   [self updateConsumer];
 }
 
-- (void)webState:(web::WebState*)webState
-    didPruneNavigationItemsWithCount:(size_t)pruned_item_count {
-  DCHECK_EQ(_webState, webState);
-  [self updateConsumer];
-}
-
 - (void)webStateDidStartLoading:(web::WebState*)webState {
   DCHECK_EQ(_webState, webState);
   [self updateConsumer];

ios/web/navigation/navigation_manager_impl_unittest.mm

@@ -83,7 +83,6 @@ class MockNavigationManagerDelegate : public NavigationManagerDelegate {
   MOCK_METHOD1(LoadCurrentItem, void(NavigationInitiationType type));
   MOCK_METHOD0(LoadIfNecessary, void());
   MOCK_METHOD0(Reload, void());
-  MOCK_METHOD1(OnNavigationItemsPruned, void(size_t));
   MOCK_METHOD1(OnNavigationItemCommitted, void(NavigationItem* item));
   MOCK_METHOD0(RemoveWebView, void());
   MOCK_METHOD4(GoToBackForwardListItem,