Update horizon local settings for Django 4

As horizon is now using Django 4 after a recent requirements update, we
need to clean our config from settings that were long deprecated and now
no longer work.

[0] https://review.opendev.org/c/openstack/horizon/+/891828
[1] https://review.opendev.org/c/openstack/horizon/+/827092

Change-Id: I47533a2ad436578c98503284c25db4fd51896506