[blob] Delete S3 objects during cleanup

Summary:
Final part of blob cleanup logic - added deletion of S3 objects.

Depends on D9352.

Test Plan:
Locally, uploaded a few items to S3 with object names hashA, hashB.
Created blob item rows in DDB: one with 'unchecked' attribute (hashA) and the other without (hashB).
No holders were given.
Ran the cleanup: only hashA file was deleted from S3.

Reviewers: michal, varun, jon

Reviewed By: michal

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9355