mac: Delete more 10.5-only code

chrome/browser/resources/help/help.css

 /* Copyright (c) 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.
  */
 
 body {
   -webkit-user-select: text;
 }
 
 #about-container {
@@ skipping 61 matching lines @@
 
 #update-status-icon.working {
   background-image: url('chrome://resources/images/throbber.svg');
 }
 
 #update-status-icon.failed {
   background-image: url('chrome://theme/IDR_UPDATE_FAIL');
   background-size: 17px;
 }
 
-#update-obsolete-os-container {
-  -webkit-margin-start: 25px;
-}
-
 #more-info-expander {
   -webkit-padding-start: 0;
 }
 
 #more-info-container.visible {
   margin-bottom: 1.5em;
 }
 
 #more-info-container {
   -webkit-transition: all 500ms linear;
   height: 0;
   margin-bottom: 0;
   overflow: hidden;
 }
 
 #last-updated-container.empty {
   visibility: hidden;
 }