I know this is late, but I recently got the fix for this myself and hope it might be useful for someone else searching for this later:
- stop the NetApp vSphere Plugin Framework service,
- stop the NetApp SnapManager for Virtual Infrastructure service
- copy the %install%/smvi/server/repository/backups.xml file
- use wordpad and edit the same file
- remove the backup entry from the "<backup uuid="" tag through to the </backup>
- restart the SMVI and NVPF services
- check and see if the backup still exists