Stop using //third_party/khronos headers in SDK

mojo/public/c/gles2/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("../../mojo_sdk.gni")
 
 config("gles2_config") {
   defines = [ "GLES2_USE_MOJO" ]
 }
 
 group("gles2") {
-  public_configs = [ "//third_party/khronos:khronos_headers" ]
   public_deps = [
     ":headers",
   ]
   deps = [
     "../../platform/native:gles2",
   ]
 }
 
 mojo_sdk_source_set("headers") {
   sources = [
     "gles2.h",
     "gles2_export.h",
     "gles2_types.h",
   ]
 
   public_configs = [ ":gles2_config" ]
 
   mojo_sdk_public_deps = [
     "mojo/public/c/environment",
     "mojo/public/c/system",
   ]
 }