Clean up PRESUBMIT.py - remove references to removed clang trybots

ash/PRESUBMIT.py

-# Copyright (c) 2012 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.
-
-"""Chromium presubmit script for src/ash
-
-See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
-for more details on the presubmit API built into gcl.
-"""
-
-def GetPreferredTryMasters(project, change):
-  return {
-    'tryserver.chromium.linux': {
-      'linux_chromium_chromeos_clang_dbg': set(['defaulttests']),
-    }
-  }