Remove Chrome OS-related GN build configuration

build/toolchain/cros/BUILD.gn

-# Copyright 2014 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("//build/toolchain/clang.gni")
-import("//build/toolchain/gcc_toolchain.gni")
-
-declare_args() {
-  # The CrOS build system supports many different kinds of targets across
-  # many different architectures. Bringing your own toolchain is also supported,
-  # so it's actually impossible to enumerate all toolchains for all targets
-  # as GN toolchain specifications.
-  # These arguments provide a mechanism for specifying your CC, CXX and AR at
-  # buildfile-generation time, allowing the CrOS build system to always use
-  # the right tools for the current target.
-  cros_target_cc = ""
-  cros_target_cxx = ""
-  cros_target_ar = ""
-}
-
-gcc_toolchain("target") {
-  assert(cros_target_cc != "", "Must provide target CC.")
-  assert(cros_target_cxx != "", "Must provide target CXX.")
-  assert(cros_target_ar != "", "Must provide target AR.")
-
-  cc = "${cros_target_cc}"
-  cxx = "${cros_target_cxx}"
-
-  ar = "${cros_target_ar}"
-  ld = cxx
-
-  toolchain_cpu = "${target_cpu}"
-  toolchain_os = "linux"
-  is_clang = is_clang
-  use_ccache = false
-  use_goma = false
-}