[swarming] Remove hard-coded shared library configs

gypfiles/isolate.gypi

 # Copyright 2015 the V8 project authors. All rights reserved.
 # 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.
 
 # This file is meant to be included into a target to provide a rule
 # to "build" .isolate files into a .isolated file.
 #
 # To use this, create a gyp target with the following form:
 # 'conditions': [
@@ skipping 62 matching lines @@
         '--config-variable', 'cfi_vptr=<(cfi_vptr)',
         '--config-variable', 'gcmole=<(gcmole)',
         '--config-variable', 'has_valgrind=<(has_valgrind)',
         '--config-variable', 'icu_use_data_file_flag=<(icu_use_data_file_flag)',
         '--config-variable', 'msan=<(msan)',
         '--config-variable', 'tsan=<(tsan)',
         '--config-variable', 'coverage=<(coverage)',
         '--config-variable', 'sanitizer_coverage=<(sanitizer_coverage)',
         '--config-variable', 'component=<(component)',
         '--config-variable', 'target_arch=<(target_arch)',
-        '--config-variable', 'use_custom_libcxx=<(use_custom_libcxx)',
         '--config-variable', 'v8_use_external_startup_data=<(v8_use_external_startup_data)',
         '--config-variable', 'v8_use_snapshot=<(v8_use_snapshot)',
       ],
       'conditions': [
         ['OS=="win"', {
           'action': [
             '--config-variable', 'msvs_version=2013',
           ],
         }, {
           'action': [
             '--config-variable', 'msvs_version=0',
           ],
         }],
       ],
     },
   ],
 }