suggested btrbk clean can delete valid backups

```
WARNING: Target subvolume "example.org:/mnt/o/btrbk/o.20230420T091826-0400" exists, but is not a receive target of "/mnt/o/btrbk/o.20230420T091826-0400"
WARNING: Please delete stray subvolumes: "btrbk clean example.org:/mnt/o/btrbk"
```

However, my main use case is to run btrbk from host1 to host2 for a few days, then switch to the opposite direction, and repeat. So, btrbk clean will kill off half my backups. When I fail a btrbk run due to a network error, I get the above suggestion. Instead, it should suggest a command that will just delete the backups that it would complain about wanting to make but not being receive targets.