This article will show the steps on how you can delete a specific folder in Roundcube interface
1) Login to your webmail with the URL as : http://www.YourDomain/webmail/ or http://www.YourDomain:2095
2) On Folder Actions icon on the corner left, click on Manage folders
3) Under the Settings tab choose on Folders
4) Under the Folders tab, choose on the Folder you wish to delete (eg Inbox_old) and click on Folder actions icon and click on Delete
5) The folder is successfully being deleted from your Roundcube interface.