Remove use_system_nspr build flag. It's not "officially" supported

base/third_party/nspr/nspr.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.
-
-{
-  'conditions': [
-    ['use_system_nspr==1', {
-      'targets': [
-        {
-          'target_name': 'nspr',
-          'type': 'none',
-          'toolsets': ['host', 'target'],
-          'variables': {
-            'headers_root_path': '.',
-            'header_filenames': [
-              'prcpucfg.h',
-              'prtime.h',
-              'prtypes.h',
-            ],
-          },
-          'includes': [
-            '../../../build/shim_headers.gypi',
-          ],
-          'direct_dependent_settings': {
-            'cflags': [
-              '<!@(pkg-config --cflags nspr)',
-            ],
-          },
-          'link_settings': {
-            'ldflags': [
-              '<!@(pkg-config --libs-only-L --libs-only-other nspr)',
-            ],
-            'libraries': [
-              '<!@(pkg-config --libs-only-l nspr)',
-            ],
-          },
-        }
-      ],
-    }],
-  ],
-}