[ios] Removes the unused tabModelClosedAllTabs: method.

BUG=911350

Change-Id: Iec791dc2bf41b10705686bf01bbadc39ccee9e9d
Reviewed-on: https://chromium-review.googlesource.com/c/1394013Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619752}

ios/chrome/browser/tabs/tab_model.mm

@@ -562,7 +562,6 @@ void RecordMainFrameNavigationMetric(web::WebState* web_state) {
 
 - (void)closeAllTabs {
   _webStateList->CloseAllWebStates(WebStateList::CLOSE_USER_ACTION);
-  [_observers tabModelClosedAllTabs:self];
 }
 
 - (void)haltAllTabs {

ios/chrome/browser/tabs/tab_model_observer.h

@@ -24,9 +24,6 @@
 // The given tab will be removed.
 - (void)tabModel:(TabModel*)model willRemoveTab:(Tab*)tab;
 
-// All tabs in the model will close.
-- (void)tabModelClosedAllTabs:(TabModel*)model;
-
 // A tab was removed at the given index.
 - (void)tabModel:(TabModel*)model
     didRemoveTab:(Tab*)tab