Revert "systemd: 239 -> 239.20181031"
Revert "systemd: 239 -> 239.20181031"

This reverts commit d1de23b8302d02d4699e884533906a3992f370b6.
The changes turned out to be too intrusive, so we'll patch instead.
Discussion: https://github.com/NixOS/systemd/pull/24

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