Fix Python 3.6 coding style error

(cherry picked from commit 87d760e)

Author: zhaoqin-github

f5/bigip/tm/ltm/test/functional/test_nat.py

@@ -332,7 +332,7 @@ def test_delete(self, request, mgmt_root, NAT):
         setup_loadable_nat_test(request, mgmt_root, NAT)
         n1 = mgmt_root.tm.ltm.nats.nat.load(name='nat1', partition='Common')
         n1.delete()
-        del(n1)
+        del n1
         with pytest.raises(HTTPError) as err:
             mgmt_root.tm.ltm.nats.nat.load(name='nat1', partition='Common')
             assert err.response.status_code == 404