backup: clean up fat manifest support

[jeffswenson]

Since #93997 we have written a file.sst, descriptor.sst and BACKUP_METADATA file. The intent was to to remove support for the "fat" BACKUP_MANIFEST file. There are still many read paths, like show backup, that only work if the fat manifest is present.

Jira issue: CRDB-46527

[blathers-crl]

Hi @jeffswenson, please add branch-* labels to identify which branch(es) this C-bug affects.

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[blathers-crl]

cc @cockroachdb/disaster-recovery