[services] Backup - Reuse remove logic

Summary:
Reuse innerRemoveItem for BackupItem`s and LogItems

Depends on D3177

Test Plan:

cd services
yarn test-backup-service

Reviewers: palys-swm, geekbrother, varun, ashoat

Reviewed By: palys-swm, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3178