Removed warning check in test_load_overlong_key

Some backends issue a warning here, others not. This is not the primary
goal of the test, so the assertion about the warning has been removed.
Thanks Carl Meyer for noticing the issue and suggesting the fix.

django/contrib/sessions/tests.py

@@ -302,11 +302,10 @@ class CacheDBSessionTests(SessionTestsMixin, TestCase):
             self.assertTrue(self.session.exists(self.session.session_key))
 
     def test_load_overlong_key(self):
-        with warnings.catch_warnings(record=True) as w:
-            warnings.simplefilter("always")
+        # Some backends might issue a warning
+        with warnings.catch_warnings():
             self.session._session_key = (string.ascii_letters + string.digits) * 20
             self.assertEqual(self.session.load(), {})
-            self.assertEqual(len(w), 1)
 
 
 @override_settings(USE_TZ=True)
@@ -352,11 +351,10 @@ class CacheSessionTests(SessionTestsMixin, unittest.TestCase):
     backend = CacheSession
 
     def test_load_overlong_key(self):
-        with warnings.catch_warnings(record=True) as w:
-            warnings.simplefilter("always")
+        # Some backends might issue a warning
+        with warnings.catch_warnings():
             self.session._session_key = (string.ascii_letters + string.digits) * 20
             self.assertEqual(self.session.load(), {})
-            self.assertEqual(len(w), 1)
 
 
 class SessionMiddlewareTests(unittest.TestCase):