Remove unused thread pool from standalone VM binary.

runtime/bin/builtin_impl_sources.gypi

 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. 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 contains all C++ sources for the dart:builtin and dart:io
 # libraries.
 {
   'sources': [
     'dartutils.cc',
     'dartutils.h',
@@ skipping 37 matching lines @@
     'process_macos.cc',
     'process_win.cc',
     'socket.cc',
     'socket.h',
     'socket_linux.cc',
     'socket_macos.cc',
     'socket_win.cc',
     'set.h',
     'set_test.cc',
     'thread.h',
-    'thread_pool.cc',
-    'thread_pool_test.cc',
     'utils.h',
     'utils_linux.cc',
     'utils_macos.cc',
     'utils_win.cc',
   ],
 }