Remove unused imports and fix some other linter errors.

recipe_engine/unittests/field_composer_test.py

 #!/usr/bin/env python
 # Copyright 2013 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 os
-import shutil
 import sys
-import tempfile
 import unittest
 
 sys.path.append(os.path.dirname(os.path.dirname(os.path.dirname(
     os.path.abspath(__file__)))))
 
 from recipe_engine import field_composer
 
 
 # Functors are compared by reference.
 BEAUTY_LAMBDA = lambda x, y: x + y + 1
@@ skipping 53 matching lines @@
   def test_compose_with_sneaky_bad_registry(self):
     """RegistryConflict must be raised when functors clash."""
     second_fc = field_composer.FieldComposer(
         {}, {'beauty': {'combine': lambda x, y: 0}})
     with self.assertRaises(field_composer.RegistryConflict):
       self.fc.compose(second_fc)
 
 
 if __name__ == '__main__':
   unittest.main()