Remove use_system_ssl GYP flag

tools/gn/secondary/crypto/ssl/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.
 
 import("flags.gni")
 import("//build/config/linux/pkg_config.gni")
 
 config("ssl_preprocessor_flags") {
   if (use_openssl) {
     defines = [ "USE_OPENSSL" ]
   } else if (use_nss) {
     defines = [ "USE_NSS" ]
   }
 }
 
-# Config for system SSL on Linux.
-if (is_linux && use_system_ssl) {
-  pkg_config("system_ssl_config") {
-    packages = [ "nss" ]
-    defines = [ "USE_SYSTEM_SSL" ]
-  }
-}
-
 # This meta-target will include the SSL library according to the build flags.
 group("metassl") {
   direct_dependent_configs = [ ":ssl_preprocessor_flags" ]
 
   if (use_openssl) {
     assert(is_linux)
     deps = "//third_party/openssl"
     use_openssl = false
-  } else if (use_system_ssl) {
-    assert(is_linux)
-    direct_dependent_configs = ":system_ssl_config"
   } else {
     deps = [ "//net/third_party/nss:ssl" ]
   }
 
   forward_dependent_configs_from = deps
 }