Code review comment for ~waveform/ubiquity:prepare-package

Re: keeping it in livecd-rootfs: we don't use livecd-rootfs for building the Pi images anymore, so if I wind up needing this as a fallback for the desktop images, it's not going to do any good there.

Having now trawled the desktop engineering Jira, it appears the plan is to deliver this via the Ubuntu Welcome application in 24.04, and things appear to be progressing well (I'll see if I can find some time to play with it in the new year). Still, I like having a plan B to fall back on, just in case (having needed them in the past!). If Ubuntu Welcome lands in time, all well and good, everyone can ignore this extra package and/or remove ubiquity entirely from the archive -- it won't make any difference. If Ubuntu Welcome *doesn't* land in time, this will be required for the Pi desktop images to work at all.