Remove GN GYP generator.

tools/gn/secondary/third_party/libusb/BUILD.gn

 # Copyright (c) 2013 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.
 
 config("libusb_config") {
   include_dirs = [
     "src/libusb",
   ]
 }
 
 static_library("libusb") {
-  external = true
   sources = [
     "src/config.h",
     "src/libusb/core.c",
     "src/libusb/descriptor.c",
     "src/libusb/hotplug.c",
     "src/libusb/hotplug.h",
     "src/libusb/interrupt.c",
     "src/libusb/interrupt.h",
     "src/libusb/io.c",
     "src/libusb/libusb.h",
@@ skipping 78 matching lines @@
       "src/libusb/os/threads_windows.h",
       "src/libusb/os/windows_common.h",
       "src/libusb/os/windows_usb.c",
       "src/libusb/os/windows_usb.h",
       "src/msvc/config.h",
       "src/msvc/inttypes.h",
       "src/msvc/stdint.h",
     ]
   }
 }