ISM: Remove systemd executor

The systemd executor is not used anymore, and the Incus executor has passed testing/ deployment. Afaik, the systemd executor is also unusable at the moment. I would like to remove the systemd executor from the codebase.

This doesn't really align with our original plan to have both executors available. However, in its current dysfunctional, untested state the executor does not contribute to the codebase/ documentation. If we need it again in the future, we can restore the code later.

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@blohmar, what is your opinion on this topic?