[keyserver] Extract deleteBackup

Summary: Working on ENG-1234, which will also require deleting old backups. Figured I'd extract this code.

Test Plan: Tested in combination with the next diff

Reviewers: atul, yayabosh

Reviewed By: yayabosh

Subscribers: palys-swm, Adrian

Differential Revision: https://phab.comm.dev/D4282