[blob-service] Require holder and blob_hash in 'remove' endpoint

Summary:
Linear task: ENG-4268.

Made the DELETE /blob HTTP endpoint require both holder and blob_hash parameters.
Also renamed delete_blob to remove_holder because this is what this endpoint actually does. The blob is deleted when the last holder is removed.

Test Plan: Tested together with the next diff

Reviewers: michal, pklatka, jon, marcin

Reviewed By: michal, jon

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8396