[services] Backup - Update proto file

Summary: We want to remove authentication from the backup service

Test Plan: None, it's just a change in the proto file.

Reviewers: palys-swm, varun, jimpo, geekbrother

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, benschac

Differential Revision: https://phabricator.ashoat.com/D3461