➖ Remove explicit defusedxml dependency

In 2024 this is no longer essential, using lxml by itself should be
sufficient.

See also the discussion on python.org:
https://discuss.python.org/t/status-of-defusedxml-and-recommendation-in-docs/34762/3

setup.cfg

@@ -39,7 +39,6 @@ install_requires =
     django-simple-certmanager
     django-solo
     lxml
-    defusedxml>=0.7.0
     furl
     maykin-python3-saml
 tests_require =