Remove unused imports and fix some other linter errors.

recipe_engine/lint_test.py

 #!/usr/bin/env python
 # Copyright 2015 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 """Tests that recipes are on their best behavior.
 
 Checks that recipes only import modules from a whitelist.  Imports are
 generally not safe in recipes if they depend on the platform, since
 e.g. you can run a recipe simulation for a Windows recipe on Linux.
 """
 
 # TODO(luqui): Implement lint for recipe modules also.
 
 from __future__ import absolute_import
 import re
-import os
-import sys
 import types
 
 
 MODULES_WHITELIST = [
   r'base64',
   r'collections',
   r'contextlib',
   r'copy',
   r'datetime',
   r'functools',
@@ skipping 40 matching lines @@
   whitelist = map(re.compile, MODULES_WHITELIST + whitelist)
 
   errors = []
   for recipe_path, recipe_name in universe_view.loop_over_recipes():
     errors.extend(
         ImportsTest(recipe_path, recipe_name, whitelist, universe_view))
 
   if errors:
     raise TestFailure('\n'.join(map(str, errors)))