`rename_part` unconditionally ran `cleanup_multipart_path([part.N, part.N.meta])`
on every disk *before* the per-disk rename fan-out. The per-disk `rename_part`
already replaces `part.N` atomically (std::fs::rename) and rewrites `part.N.meta`,
so the pre-delete is redundant — and destructive: it removed an already-committed
(ACKed) part on all disks before the new rename landed, so a re-upload of the same
part that then failed write quorum destroyed the committed part outright.
Remove the pre-rename cleanup and rely on the atomic rename to overwrite in place;
the quorum-failure rollback below is unchanged. No behaviour change on the success
paths (first upload / retry both end with part.N replaced), verified by the
existing multipart suite.
Refs backlog#799 (B4). The remaining half of B4 — serializing concurrent
same-part uploads with an uploadId-scoped lock so two generations can't be mixed
across disks — is a larger concurrency change tracked as a follow-up on the issue.
