remove restic from crw-operator builds (removed as of Che 7.43.x) & backup-rhel8 from CRW 2.16 (no longer supported)

As of Che 7.43, there's no more restic in the che-operator Dockerfile and restic sources have been removed from che-operator/pkgs/backup_servers/.

So downstream we need to do similar with the build / sync scripts.

This also means we can remove the backup-rhel8 image from the CRW 2.16 image payload, as there's no more backup code in the operator.