NaCl/GYP: remove references to prep_toolchain.

ppapi/native_client/src/untrusted/irt_stub/irt_stub.gyp

 # Copyright (c) 2011 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.
 
 {
   'includes': [
     '../../../../../build/common_untrusted.gypi',
   ],
   'targets' : [
     {
       'target_name': 'ppapi_stub_lib',
       'type': 'none',
       'variables': {
         'nlib_target': 'libppapi_stub.a',
         'build_glibc': 1,
         'build_newlib': 1,
         'build_pnacl_newlib': 1,
       },
       'include_dirs': [
         '../../../..',
       ],
       'sources': [
         'plugin_main_irt.c',
         'ppapi_plugin_main.c',
         'ppapi_plugin_start.c',
         'thread_creator.c'
       ],
-      'dependencies': [
-        '<(DEPTH)/native_client/tools.gyp:prep_toolchain',
-      ],
     },
   ],
 }