Add explicit mechanism to remove rollback deployment

[jcpunk]

rpm-ostree cleanup provides a way to remove old data. Is there a bootc integration/automation/script/etc?

[cgwalters]

bootc automatically prunes data not referenced by a deployment, the same as rpm-ostree. However, we don't yet offer a CLI or API to explicitly remove the pending or rollback state, but we should.

(We do interoperate though today, using rpm-ostree cleanup -b will remove the rollback and bootc handles that, as does the lower level ostree admin undeploy, but that won't automatically GC container layers)

This relates a lot to #904 on the opposite end.

I am retitling this issue under the assumption that your RFE is a way to remove the rollback, is that correct?

[jcpunk]

I'm primarily interested in getting the current behavior documented.

RFEs take a while and may require some design work...

that won't automatically GC container layers

Eventually I'd love that too! My projected "nodes" look like they may have a 25 year field lifetime, so garbage collection will eventually be very important along side historical records.