Get rid of system_export.h, etc.

mojo/edk/util/BUILD.gn

 # 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.
 
 import("../mojo_edk.gni")
 
 mojo_edk_source_set("util") {
   sources = [
     "make_unique.h",
     "scoped_file.h",
   ]
 
-  defines = [ "MOJO_SYSTEM_IMPLEMENTATION" ]
-
   mojo_edk_configs = [ "mojo/edk/system:system_config" ]
 
   deps = [
     "//base",
   ]
 }