Remove "Windows toolchain" step from DEPS. May only be used by gyp.

DEPS

 # 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.
 
 # IMPORTANT:
 # Before adding or updating dependencies, please review the documentation here:
 # https://github.com/dart-lang/sdk/wiki/Adding-and-Updating-Dependencies
 
 vars = {
   # The dart_root is the root of our sdk checkout. This is normally
@@ skipping 454 matching lines @@
     'name': 'download_android_tools',
     'pattern': '.',
     'action': ['python', 'sdk/tools/android/download_android_tools.py'],
   },
   {
     'name': 'buildtools',
     'pattern': '.',
     'action': ['python', 'sdk/tools/buildtools/update.py'],
   },
   {
-    # Update the Windows toolchain if necessary.
-    'name': 'win_toolchain',
-    'pattern': '.',
-    'action': ['python', 'sdk/build/vs_toolchain.py', 'update'],
-  },
-  {
     "pattern": ".",
     "action": ["python", Var("dart_root") + "/tools/generate_buildfiles.py"],
   },
 ]