Remove mac_x64_rel from trybot defaults.

As this bot was removed in https://codereview.chromium.org/1613013003/

TBR=fbarchard@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1615793002 .

PRESUBMIT.py

@@ -28,7 +28,6 @@ def GetPreferredTryMasters(project, change):
     'win_x64_rel',
     'mac',
     'mac_rel',
-    'mac_x64_rel',
     'ios',
     'ios_rel',
     'ios_arm64',