Remove GN GYP generator.

tools/gn/secondary/third_party/mach_override/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.
 
 static_library("libudis86") {
-  external = true
   sources = [
     "libudis86/decode.c",
     "libudis86/decode.h",
     "libudis86/extern.h",
     "libudis86/input.c",
     "libudis86/input.h",
     "libudis86/itab.c",
     "libudis86/itab.h",
     "libudis86/syn-att.c",
     "libudis86/syn-intel.c",
     "libudis86/syn.c",
     "libudis86/syn.h",
     "libudis86/types.h",
     "libudis86/udint.h",
     "libudis86/udis86.c",
     "udis86.h",
   ]
   defines = [
     "HAVE_ASSERT_H",
     "HAVE_STRING_H",
   ]
 }
 
 static_library("mach_override") {
-  external = true
   sources = [
     "mach_override.c",
     "mach_override.h",
   ]
   deps = [
     ":libudis86",
   ]
 }