Fix incremental and archive backup removal on delete command

The previous algorithm was smart enough to remove full backups older
than the given number of generations, but not enough to remove
incremental and archive backups. This resulted in keeping in the backup
list a set of incremental and archive backups older than the latest
full backup allowed. As it is useless to keep them, the deletion
algorithm is made smarter to take that into account and remove all of
them cleanly only when necessary.