dracut: clean up /boot/ignition when done

[bgilbert]

Avoid leaking an injected Ignition config into the real root once we're done with it.

For coreos/fedora-coreos-tracker#889. Requires coreos/fedora-coreos-config#1092.

[jlebon]

Hmm, IMO I think we should go the other way and remove ignition-setup-user.service from here:

1. Out-of-band methods for passing Ignition configs is very distro-specific. Here for example, we're encoding the fact that the boot partition has label boot, which while a sensible label, is an odd thing to have Ignition dictate just so it can support a secondary input path.
2. Mounting the boot partition, especially in read-write mode, is something that may require careful ordering and benefits greatly from distro centralization. I'd prefer having as few OS components as possible care about boot partitions so it's easier to manage at the integration level (right now for example in FCOS/RHCOS, we need a doc to keep track of it all).

That said, if we do decide to still keep it here, then I agree it'd make sense to have the deletion happen here too.

[bgilbert]

Yeah, I think that makes sense. I'll close this and post PRs to move ignition-setup-user.service out of this repo.

[bgilbert]

ignition-setup-user.service removal in #1248.