Eliminate the 'build_ppapi_ipc_proxy_untrusted' build flag for untrusted PPAPI proxy builds.

ppapi/ppapi_ipc_proxy_untrusted.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': {
     'chromium_code': 1,
   },
   'includes': [
     '../native_client/build/untrusted.gypi',
@@ skipping 29 matching lines @@
             '../gpu/gpu_untrusted.gyp:gles2_cmd_helper_untrusted',
             '../gpu/gpu_untrusted.gyp:gpu_ipc_untrusted',
             '../ipc/ipc_untrusted.gyp:ipc_untrusted',
             '../ppapi/ppapi_shared_untrusted.gyp:ppapi_shared_untrusted',
           ],
         },
       ],
     }],
   ],
 }