Remove old references to cpu_arch, os from GN files.

build/toolchain/win/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.
 
 declare_args() {
   # Path to the directory containing the VC binaries for the right
   # combination of host and target architectures. Currently only the
   # 64-bit host toolchain is supported, with either 32-bit or 64-bit targets.
   # If vc_bin_dir is not specified on the command line (and it normally
   # isn't), we will dynamically determine the right value to use at runtime.
@@ skipping 183 matching lines @@
     tool("copy") {
       command =
           "$python_path gyp-win-tool recursive-mirror {{source}} {{output}}"
       description = "COPY {{source}} {{output}}"
     }
 
     # When invoking this toolchain not as the default one, these args will be
     # passed to the build. They are ignored when this is the default toolchain.
     toolchain_args() {
       current_cpu = invoker.current_cpu
-
-      # TODO(dpranke): cpu_arch is here for backwards compatibility.
-      cpu_arch = current_cpu
     }
   }
 }
 
 # TODO(dpranke): Declare both toolchains all of the time when we
 # get it sorted out how we want to support them both in a single build.
 # Right now only one of these can be enabled at a time because the
 # runtime libraries get copied to root_build_dir and would collide.
 if (current_cpu == "x86") {
   msvc_toolchain("32") {
     environment = "environment.x86"
 
     current_cpu = "x86"
   }
 }
 
 if (current_cpu == "x64") {
   msvc_toolchain("64") {
     environment = "environment.x64"
 
     current_cpu = "x64"
   }
 }