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

ash/PRESUBMIT.py

Index: ash/PRESUBMIT.py
diff --git a/ash/PRESUBMIT.py b/ash/PRESUBMIT.py
deleted file mode 100644
index 9132e99a3dd71ca17197dd68face21caa34e430a..0000000000000000000000000000000000000000
--- a/ash/PRESUBMIT.py
+++ /dev/null
@@ -1,16 +0,0 @@
-# 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']),
-    }
-  }