Removes unused test class member and corresponding import.

Leased sockets is from an older architecture.

tests/test_exception_handling.py

@@ -14,7 +14,6 @@
 # limitations under the License.
 
 from unittest2 import TestCase
-from collections import defaultdict
 
 import zipline.utils.simfactory as simfactory
 from zipline.test_algorithms import (
@@ -39,8 +38,6 @@ EXTENDED_TIMEOUT = 90
 
 class ExceptionTestCase(TestCase):
 
-    leased_sockets = defaultdict(list)
-
     def setUp(self):
         self.zipline_test_config = {
             'sid': 133,