Fix a test for MongoDB 3.2 change

Author: behackett

test/test_read_preferences.py

@@ -140,7 +140,6 @@ def test_reads_from_secondary(self):
 
  # Test some "magic" namespace helpers.
  self.assertIsNotNone(db.current_op())
- client.unlock() # No error.
 
 
 class TestReadPreferences(TestReadPreferencesBase):