Remove mac_x64_rel from trybot defaults.

PRESUBMIT.py

 # Copyright 2014 The LibYuv Project Authors. All rights reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS. All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import re
 import sys
@@ skipping 10 matching lines @@
 
 # pylint: disable=W0613
 def GetPreferredTryMasters(project, change):
   files = change.LocalPaths()
   bots = [
     'win',
     'win_rel',
     'win_x64_rel',
     'mac',
     'mac_rel',
-    'mac_x64_rel',
     'ios',
     'ios_rel',
     'ios_arm64',
     'ios_arm64_rel',
     'mac_asan',
     'linux',
     'linux_rel',
     'linux_memcheck',
     'linux_tsan2',
     'linux_asan',
     'android',
     'android_rel',
     'android_clang',
     'android_arm64',
   ]
   if not files or all(re.search(r'[\\/]OWNERS$', f) for f in files):
     return {}
   return GetDefaultTryConfigs(bots)