[Backup] Old backup cleanup
Summary:
ENG-4501
For the initial version of the backup service we only want to keep one backup for every user. In this diff we remove all old backups after every new backup is successfuly uploaded.
Depends on D8967
Test Plan: Run integration tests
Reviewers: bartek, patryk, kamil, jon
Reviewed By: bartek
Subscribers: varun, ashoat, tomek
Differential Revision: https://phab.comm.dev/D8969