Remove windows/mac/ios specific code from //ui

ui/platform_window/win/BUILD.gn

Index: ui/platform_window/win/BUILD.gn
diff --git a/ui/platform_window/win/BUILD.gn b/ui/platform_window/win/BUILD.gn
deleted file mode 100644
index 47541eafdcfc1e4aab9bc3f4f8eb460d8a9491a3..0000000000000000000000000000000000000000
--- a/ui/platform_window/win/BUILD.gn
+++ /dev/null
@@ -1,22 +0,0 @@
-# Copyright 2014 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.
-
-component("win") {
-  output_name = "win_window"
-
-  deps = [
-    "//base",
-    "//skia",
-    "//ui/events",
-    "//ui/platform_window",
-  ]
-
-  defines = [ "WIN_WINDOW_IMPLEMENTATION" ]
-
-  sources = [
-    "win_window.cc",
-    "win_window.h",
-    "win_window_export.h",
-  ]
-}