headless: Replace is_headless gn setting with an args template

build/config/headless_build.gni

Index: build/config/headless_build.gni
diff --git a/build/config/headless_build.gni b/build/config/headless_build.gni
deleted file mode 100644
index 1788b25ab804dfe5810a561fe2faff54c0704661..0000000000000000000000000000000000000000
--- a/build/config/headless_build.gni
+++ /dev/null
@@ -1,8 +0,0 @@
-# Copyright 2015 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.
-
-declare_args() {
-  # Configure the build for headless mode. See crbug.com/546953.
-  is_headless = false
-}