It is part of LOCAL backup... When in Console Mode it is part of the Settings (Gearbox-icon), select backup. It is also a stand-alone function (called Backup System), but then you need to log in separately.
When logged in into Backup, select New Period, and then select via UI, or manually set Start- and End-date/time. Then, select the Delete option. This will remove the footage and all related administration. You will then no longer be able to retrieve the footage.
Quickest is via Console, Settings, Backup....
Note: Delete is only allowed on Local Box, so not from remote... Backup is allowed everywhere. Also Playback has backup-functionality, but more limited than the 'real' backup. No delete, no multi-timelines, and a few more...