Remove use_system_ssl GYP flag

tools/gn/secondary/crypto/ssl/flags.gni

 # 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.
 
 # This file declares build flags for the SSL library configuration.
 
 declare_args() {
   # Use OpenSSL instead of NSS. This is used for Android and is experimental
   # in other cases (see http://crbug.com/62803).
   use_openssl = false
-
-  # Use the built-in system SSL library rather than our tree's libnss/OpenSSL.
-  use_system_ssl = false
 }
 
 if (is_linux && !use_openssl) {
   use_nss = true
 } else {
   use_nss = false
 }