Remove UserTasksMax in logind.conf

    logind-compat-tasks-max-generator[26881]: /etc/systemd/logind.conf:36: Support for option UserTasksMax= is deprecated and will be removed.

While systemd thankfully ignores non-existent keys it still leads to warnings - plus not having task limits set after upgrades might be unpleasant. Remove this and make sure task limits still are sensible.