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

chrome/browser/policy/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 chrome/browser/policy.
 
 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_rel': set(['defaulttests']),
-      'linux_chromium_chromeos_clang_dbg': set(['defaulttests']),
     }
   }