Simplify start-systemd-desktop-target

Some of the tweaks in config/chroot_local-includes/usr/local/lib/start-systemd-desktop-target were needed on Jessie, but they might not be needed anymore on newer Debian releases. Let’s keep only the ones we still need.

Related issues

Blocks #16209

Original created by @intrigeri on 16968 (Redmine)

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