Remove presubmit bot from 'git cl try'.

'git cl try' is often executed before an lgtm was given - in
this case the bot always fails.

TBR=jochen@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/820463004

Cr-Commit-Position: refs/heads/master@{#26182}

PRESUBMIT.py

@@ -243,7 +243,6 @@ def CheckChangeOnCommit(input_api, output_api):
 def GetPreferredTryMasters(project, change):
   return {
     'tryserver.v8': {
-      'v8_presubmit': set(['defaulttests']),
       'v8_linux_rel': set(['defaulttests']),
       'v8_linux64_rel': set(['defaulttests']),
       'v8_linux64_asan_rel': set(['defaulttests']),