Remove obsolete includes of tab_private.h.

The tab_private.h file exposes Tab private APIs that are deprecated
so remove includes that are no longer necessary.

Bug: 620465
Change-Id: I7afca1fb2346405c06f4a7eae50041aa9891ae91
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Reviewed-on: https://chromium-review.googlesource.com/806176Reviewed-by: Elodie Banel <lod@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521335}

ios/chrome/browser/tabs/legacy_tab_helper.mm

@@ -5,7 +5,6 @@
 #import "ios/chrome/browser/tabs/legacy_tab_helper.h"
 
 #import "ios/chrome/browser/tabs/tab.h"
-#import "ios/chrome/browser/tabs/tab_private.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."

ios/chrome/browser/tabs/tab_model_closing_web_state_observer.mm

@@ -12,7 +12,6 @@
 #import "ios/chrome/browser/tabs/legacy_tab_helper.h"
 #import "ios/chrome/browser/tabs/tab.h"
 #import "ios/chrome/browser/tabs/tab_model.h"
-#import "ios/chrome/browser/tabs/tab_private.h"
 #import "ios/chrome/browser/web_state_list/web_state_list.h"
 #import "ios/web/public/navigation_item.h"
 #import "ios/web/public/navigation_manager.h"