Remove //mojo/platform_handle

mojo/mojo_platform_handle.gyp

-# Copyright 2016 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': {
-    # This turns on e.g. the filename-based detection of which
-    # platforms to include source files on (e.g. files ending in
-    # _mac.h or _mac.cc are only compiled on MacOSX).
-    'chromium_code': 1,
-  },
-  'targets': [
-    {
-      # GN version: //mojo/platform_handle:platform_handle/platform_handle_impl
-      'target_name': 'platform_handle',
-      'type': 'none',
-    }
-  ]
-}