[Merge] ~mwhudson/livecd-rootfs/+git/livecd-rootfs:lp-2028862 into livecd-rootfs:ubuntu/master

[Michael Hudson-Doyle]

A very long time ago I think we might have had issues with subiquity starting before plymouth cleared the splash screen. But we disabled the splash on the server ISO at least 5 years ago I think.  Not sure why systemd-user-sessions is there -- it's been around since the first subiquqity-related unit was added in 2017. Given that that this version wasn't even run by a snapd generated unit, I think there's a fair chance it's not needed any more...

As to why we set After=snapd.seeded.service, no idea. The services aren't going to be started at all until snapd is up and running...
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https://code.launchpad.net/~mwhudson/livecd-rootfs/+git/livecd-rootfs/+merge/448655
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