Remove LOG line checking from presubmit

As discussed at https://groups.google.com/d/msg/v8-dev/4EWhm5hltUs/daJgVxHHBQAJ,
this check is not useful, and mostly serves to be a speed bump that simply forces
developers to add "LOG=N" to their commits.

The ChangeLog is already sparse enough (see detail stats at
https://groups.google.com/d/msg/v8-dev/4EWhm5hltUs/Z276qyh8CQAJ) that it's
very unlikely to be a useful signal compared to reading the commit log directly.

LOG=y
NOTRY=true

Review-Url: https://codereview.chromium.org/1956993002
Cr-Commit-Position: refs/heads/master@{#36147}

PRESUBMIT.py

@@ -242,32 +242,15 @@ def _SkipTreeCheck(input_api, output_api):
   return input_api.environ.get('PRESUBMIT_TREE_CHECK') == 'skip'
 
 
-def _CheckChangeLogFlag(input_api, output_api, warn):
-  """Checks usage of LOG= flag in the commit message."""
-  results = []
-  if (input_api.change.BUG and input_api.change.BUG != 'none' and
-      not 'LOG' in input_api.change.tags):
-    text = ('An issue reference (BUG=) requires a change log flag (LOG=). '
-            'Use LOG=Y for including this commit message in the change log. '
-            'Use LOG=N or leave blank otherwise.')
-    if warn:
-      results.append(output_api.PresubmitPromptWarning(text))
-    else:
-      results.append(output_api.PresubmitError(text))
-  return results
-
-
 def CheckChangeOnUpload(input_api, output_api):
   results = []
   results.extend(_CommonChecks(input_api, output_api))
-  results.extend(_CheckChangeLogFlag(input_api, output_api, True))
   return results
 
 
 def CheckChangeOnCommit(input_api, output_api):
   results = []
   results.extend(_CommonChecks(input_api, output_api))
-  results.extend(_CheckChangeLogFlag(input_api, output_api, False))
   results.extend(input_api.canned_checks.CheckChangeHasDescription(
       input_api, output_api))
   if not _SkipTreeCheck(input_api, output_api):