Remove unused imports and fix some other linter errors.

recipe_engine/unittests/autoroll_test.py

 #!/usr/bin/env python
 # Copyright 2016 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.
 
-import json
-import os
-import subprocess
 import sys
 import unittest
 
 import test_env
-import mock
 
-from recipe_engine import package
 from recipe_engine import autoroll
 
 class FakeCandidate(object):
   def __init__(self, data, projects=None):
     self._projects = projects
     self._data = data
 
   def get_affected_projects(self):
     return self._projects
 
@@ skipping 27 matching lines @@
   def test_ignore_some_candidates(self):
     self.assertEqual(
         ['build', ],
         autoroll.process_rejected([
             FakeCandidate('build', ['build']),
             FakeCandidate('depot_tools', ['depot_tools']),
         ], ['build']))
 
 if __name__ == '__main__':
   sys.exit(unittest.main())