Is this possible in the section backupConfiguration inside icCube.xml?
As an alternative, is it possible to create some sort of Schedulertask to delete old backups? Or is there a possibility to trigger this via the XMLA-interface?
You can configure icCube to keep the last backup (see backupHistory in inCube.xml). Otherwise, you can configure a cron task to cleanup the backup directory according to your own requirements.