hidden files and delete backup

When deleting backup produced by version older than 2.3.0, it is possible to get the following error:
```
WARNING: Skip hidden file: '/backup/postgresql/backup/backups/instance/Q9DLCT/database/.postgresql.conf.swp'
ERROR: Cannot remove directory "/backup/postgresql/backup/backups/instance/Q9DLCT/database": Directory not empty
```

Reported by Alex Ignatov.