Remove unused imports and fix some other linter errors.

recipe_engine/stream_logdog.py

Index: recipe_engine/stream_logdog.py
diff --git a/recipe_engine/stream_logdog.py b/recipe_engine/stream_logdog.py
index 8c8c69703ccfd7efa4a0a98496f5b3f8237e8827..1e1e12b9943e436bb84a6e78760c9c85ef484ee5 100644
--- a/recipe_engine/stream_logdog.py
+++ b/recipe_engine/stream_logdog.py
@@ -7,21 +7,16 @@ protobuf.
 """
 
 import collections
-import contextlib
 import copy
 import datetime
 import functools
-import itertools
 import os
 import threading
 import sys
 
 from . import env
 from . import stream
-from . import util
 
-import google.protobuf.message
-import google.protobuf.timestamp_pb2 as timestamp_pb2
 import libs.logdog.bootstrap
 import libs.logdog.stream
 import libs.logdog.streamname