Remove unused cm variable

Author: Bolotnyj

Lib/test/test_filecmp.py

@@ -391,7 +391,6 @@ def _assert_report(self, dircmp_report, expected_report_lines):
 
 class DirCmpImplementationTests(unittest.TestCase):
     def setUp(self):
-        cm = os_helper.temp_dir()
         tmpdir = self.enterContext(os_helper.temp_dir())
 
         self.base_test_dir = os.path.join(tmpdir, 'test_filecmp_working_dir')