Remove unused cygwin build step

build/win/system.gyp

-# Copyright (c) 2009 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.
-
-{
-  'targets': [
-    {
-      'target_name': 'cygwin',
-      'type': 'none',
-      'actions': [
-        {
-          'action_name': 'setup_mount',
-          'msvs_cygwin_shell': 0,
-          'inputs': [
-            '../../third_party/cygwin/setup_mount.bat',
-          ],
-          # Visual Studio requires an output file, or else the
-          # custom build step won't run.
-          'outputs': [
-            '../../third_party/cygwin/_always_run_setup_mount.marker',
-          ],
-          'action': ['', '<@(_inputs)'],
-        },
-      ],
-    },
-  ],
-}