Remove GN GYP generator.

tools/gn/secondary/third_party/zlib/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("zlib_config") {
   include_dirs = [ "." ]
 }
 
 static_library("zlib") {
-  external = true
-
   if (!is_win) {
     # Don't stomp on "libzlib" on other platforms.
     output_name = "chrome_zlib"
   }
 
   sources = [
     "adler32.c",
     "compress.c",
     "crc32.c",
     "crc32.h",
@@ skipping 22 matching lines @@
     "zutil.h",
   ]
 
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
 
   direct_dependent_configs = [ ":zlib_config" ]
 }
 
 static_library("minizip") {
-  external = true
   sources = [
     "contrib/minizip/ioapi.c",
     "contrib/minizip/ioapi.h",
     "contrib/minizip/iowin32.c",
     "contrib/minizip/iowin32.h",
     "contrib/minizip/unzip.c",
     "contrib/minizip/unzip.h",
     "contrib/minizip/zip.c",
     "contrib/minizip/zip.h",
   ]
@@ skipping 15 matching lines @@
   }
 
   deps = [ ":zlib" ]
 
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
   direct_dependent_configs = [ ":zlib_config" ]
 }
 
 static_library("zip") {
-  external = true
   sources = [
     "google/zip.cc",
     "google/zip.h",
     "google/zip_internal.cc",
     "google/zip_internal.h",
     "google/zip_reader.cc",
     "google/zip_reader.h",
   ]
   deps = [
     ":minizip",
     "//base",
   ]
 }