Remove Windows-related GN build configuration

build/toolchain/goma.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.
 
 # Defines the configuration of Goma.
 #
 # This is currently designed to match the GYP build exactly, so as not to break
 # people during the transition.
 
 declare_args() {
   # Set to true to enable distributed compilation using Goma.
   use_goma = false
 
-  # Set the default value based on the platform.
-  if (is_win) {
-    # Absolute directory containing the Goma source code.
-    goma_dir = "C:\goma\goma-win"
-  } else {
-    # Absolute directory containing the Goma source code.
-    goma_dir = getenv("HOME") + "/goma"
-  }
+  # Absolute directory containing the Goma source code.
+  goma_dir = getenv("HOME") + "/goma"
 }