Drop systemd service disablement workaround

Ignition supports modifying service enablement by injecting preset files. However, disabling services hasn't worked because, until recently, systemd has been only using preset files to enable services and not disable them. As a result, we've added some code in Ignition to workaround this.

Now, systemd has made this behaviour configurable so that preset files are also used to disable systemd units, and Fedora rawhide has started to opt in. The eventual goal is to make it the default upstream.

As a result, we should be able to remove our workaround at some point. However, we need to be aware of other distributions where Ignition may rev faster than systemd. There may be also other non-Fedora-based distros which may not have opted in (but we could document that we expect them to opt in if upstream takes too long to change defaults).

Review the workaround introduced in Ignition PR #1352 and the linked systemd preset behavior changes first. Determine which distributions support preset-based disabling before removing the workaround, and document any required systemd behavior or distribution expectations. Done means the workaround is safely removed or its compatibility constraints are recorded.