Delete a variant for a benchmark. The user must have delete permissions for the project. All reports that use this variant must be deleted first!

A benchmark's empty variant cannot be deleted. The empty variant is structural: every benchmark is born with exactly one, and report ingest treats a missing empty variant as data corruption rather than a variant to mint. Deleting it would manufacture exactly that corruption, so the request is refused. Archiving the empty variant stays allowed, because a later report revives it the same way it revives any other archived variant.