Through some random machinations, software-properties-common now has a dependency on systemd. This breaks the built docker image as plugins will think systemd is available, so `dokku plugin:install --core` will reference that over sv.
Since we don't manage ppas in Dokku anymore - the only use was for an nginx ppa so we could use a newer Nginx - this can be safely removed.
Also remove some other stuff related to systemd that aren't required anymore.
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