Removed references to linux_gpu, which has been removed

gpu/PRESUBMIT.py

 # Copyright (c) 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.
 
 """Top-level presubmit script for gpu/.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details about the presubmit API built into depot_tools.
 """
 
 def GetPreferredTryMasters(project, change):
   return {
     'tryserver.chromium.gpu': {
-      'linux_gpu': set(['defaulttests']),
       'win_gpu': set(['defaulttests']),
     }
   }