Clean up Mojo C API dependent targets

mojo/message_pump/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("//testing/test.gni")
 
 component("message_pump") {
   sources = [
     "handle_watcher.cc",
     "handle_watcher.h",
     "message_pump_mojo.cc",
     "message_pump_mojo.h",
     "message_pump_mojo_handler.h",
     "time_helper.cc",
     "time_helper.h",
   ]
 
   defines = [ "MOJO_MESSAGE_PUMP_IMPLEMENTATION" ]
 
   public_deps = [
     "//base",
     "//mojo/public/cpp/system",
   ]
 
   deps = [
     "//base/third_party/dynamic_annotations",
-    "//mojo/public/c/system:for_component",
   ]
 }