Removed references to linux_gpu, which has been removed

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.
 
 """Top-level presubmit script for Chromium.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details about the presubmit API built into gcl.
 """
 
@@ skipping 1578 matching lines @@
 
 
 def GetTryServerMasterForBot(bot):
   """Returns the Try Server master for the given bot.
 
   It tries to guess the master from the bot name, but may still fail
   and return None.  There is no longer a default master.
   """
   # Potentially ambiguous bot names are listed explicitly.
   master_map = {
-      'linux_gpu': 'tryserver.chromium.gpu',
       'win_gpu': 'tryserver.chromium.gpu',
       'chromium_presubmit': 'tryserver.chromium.linux',
       'blink_presubmit': 'tryserver.chromium.linux',
       'tools_build_presubmit': 'tryserver.chromium.linux',
   }
   master = master_map.get(bot)
   if not master:
     if 'gpu' in bot:
       master = 'tryserver.chromium.gpu'
     elif 'linux' in bot or 'android' in bot or 'presubmit' in bot:
@@ skipping 84 matching lines @@
         if 'presubmit' in builder:
           builders[master].pop(builder)
 
   # Match things like path/aura/file.cc and path/file_aura.cc.
   # Same for chromeos.
   if any(re.search(r'[\\\/_](aura|chromeos)', f) for f in files):
     tryserver_linux = builders.setdefault('tryserver.chromium.linux', {})
     tryserver_linux['linux_chromium_chromeos_asan_rel_ng'] = ['defaulttests']
 
   return builders