How to remove stray subvolumes?

Hi!

I've interrupted btrbk's operation at one point and now very time I use it is says this:
```
WARNING: Target subvolume "/mnt/backup/data/data.20190526" exists, but is not a receive target of "/mnt/data/backup_snapshots/data.20190526"
WARNING: Please delete stray subvolume ("btrbk clean"): /mnt/backup/data/data.20190526
WARNING: Skipping backup of: /mnt/data/backup_snapshots/data.20190526
Creating backup: /mnt/backup/data/data.20190602
```

I've tried this:
```
$ btrbk clean /mnt/backup/data/data.20190526
ERROR: Command line argument does not match any volume, subvolume, target or group declaration: /mnt/backup/data/data.20190526
```
What can I do to clean this up safely?