Add the ability to prune old backups (where the source volume doesn't exist anymore)

Hi,

I've got the following use case:

Over the years I've backupped my old PC using btrfs ( 7d 4w 12m *y). I've replaced the PC with a laptop, moving most (but not all) data to the laptop which now creates the "new" backups.

Now, as the laptop creates it's backups I would like to prune the old PC laptops, retaining them according to the preserve policy, but I haven't found a way to do it - using a "dummy" volume will result in an error. So it would be quite useful to have the abilty of a "dummy/prune" volume which won't fetch anything but just prunes the old backups

The issue names no files, tests, or entry points. Begin by tracing how a configured volume is validated and how preserve-policy pruning is invoked, then confirm that a dummy or prune-only volume can remove backups whose source volume is absent without fetching data.

Written by the indexing model from the issue text.