Revert of [ObjC ARC] Converts ios/chrome/app to ARC.

ios/chrome/app/BUILD.gn

 # Copyright 2015 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.
 
 source_set("app") {
   sources = [
     "application_delegate/memory_warning_helper.h",
     "application_delegate/memory_warning_helper.mm",
     "deferred_initialization_runner.h",
     "deferred_initialization_runner.mm",
     "safe_mode_crashing_modules_config.h",
     "safe_mode_crashing_modules_config.mm",
     "safe_mode_util.cc",
     "safe_mode_util.h",
   ]
 
   deps = [
     "//base",
     "//ios/chrome/browser",
   ]
 
   libs = [
     "Foundation.framework",
     "UIKit.framework",
   ]
-
-  configs += [ "//build/config/compiler:enable_arc" ]
 }