Remove obsolete code in TabModelOrderController.

ios/chrome/browser/tabs/tab_model.h

 // Copyright 2012 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.
 
 #ifndef IOS_CHROME_BROWSER_TABS_TAB_MODEL_H_
 #define IOS_CHROME_BROWSER_TABS_TAB_MODEL_H_
 
 #import <Foundation/Foundation.h>
 #import <UIKit/UIKit.h>
 
@@ skipping 146 matching lines @@
 // Placing anew tab in its proper location along the tab strip is based on the
 // tab's 'opener' tab, the tab from which the new tab was opened. For the user,
 // that does not mean the tab in UI terms, but the page from which the new tab
 // is opened. Currently, to best provide this behavior, the session ID of the
 // tab is supplemented with its navigation index. If both those items match, it
 // is assumed that the tabs belong to the same owner's group. Note that using
 // navigation index is a limited solution, and may in some cases be incorrect,
 // as forward or back navigations (incrementing/decrementing the navigation
 // index) may result in incorrect tab pairings.
 
-// Returns the first tab in the model opened by the specified tab at its current
-// navigation index. The search starts at the beginning of the list and stops at
-// |tab|. Returns nil if no tab meets these constraints.
-- (Tab*)firstTabWithOpener:(Tab*)tab;
 // Returns the last tab in the model opened by the specified tab at its current
 // navigation index. The search starts at |tab|. Returns nil if no tab meets
 // these constraints.
 - (Tab*)lastTabWithOpener:(Tab*)tab;
 
 // Returns the tab which opened this tab, or nil if it's not a child.
 - (Tab*)openerOfTab:(Tab*)tab;
 
 // Add/modify tabs.
 
@@ skipping 126 matching lines @@
 // TYPED.
 - (Tab*)insertTabWithURL:(const GURL&)URL
                 referrer:(const web::Referrer&)referrer
               windowName:(NSString*)windowName
                   opener:(Tab*)parentTab
                  atIndex:(NSUInteger)index;
 
 @end
 
 #endif  // IOS_CHROME_BROWSER_TABS_TAB_MODEL_H_