delete all children's previews when deleting a folder

supersedes https://github.com/owncloud/core/pull/10634

please review @LukasReschke @DeepDiver1975 
That's intended, because I add elements to $children over time: https://github.com/owncloud/core/pull/11741/files#diff-18fbdb7eabef5315cacf23e5a9dd6970R823

That's actually why I use for instead of foreach