[keyserver] Delete backup-phabricator.sh

Summary: Deletes backup-phabricator.sh, per @ashoat's comment on Linear here.

Test Plan: I no longer see the file

Reviewers: ashoat, atul

Reviewed By: ashoat, atul

Subscribers: palys-swm, Adrian, ashoat

Differential Revision: https://phab.comm.dev/D4397