remove -Wno-header-guard workaround for old versions of NSS

build/linux/system.gyp

 # 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.
 
 {
   'variables': {
     'conditions': [
       ['sysroot!=""', {
         'pkg-config': '<(chroot_cmd) ./pkg-config-wrapper "<(sysroot)" "<(target_arch)" "<(system_libdir)"',
       }, {
@@ skipping 1186 matching lines @@
               },
               'link_settings': {
                 'ldflags': [
                   '<!@(<(pkg-config) --libs-only-L --libs-only-other nss)',
                 ],
                 'libraries': [
                   '<!@(<(pkg-config) --libs-only-l nss | sed -e "s/-lssl3//")',
                 ],
               },
             }],
-            ['use_openssl==0 and clang==1', {
-              'direct_dependent_settings': {
-                'cflags': [
-                  # There is a broken header guard in /usr/include/nss/secmod.h:
-                  # https://bugzilla.mozilla.org/show_bug.cgi?id=884072
-                  '-Wno-header-guard',
-                ],
-              },
-            }],
           ]
         }],
       ],
     },
   ],
 }