refactor(s3-client): remove the superseded per-algorithm checksum plumbing
Deletes the write-only RequestMetadata.add_crc pipeline (assigned but never read since the port), the dead MinIO-parity Checksum constructors and CompletePart accessor, and key_capitalized (identical to key). The five hand-rolled x-amz-checksum-* response-header if-lets in the streaming and multipart paths collapse into one checksum_header_value helper, ChecksumMode's inherent to_string becomes a Display impl, and checksum.rs drops its file-wide allow blanket now that the file is lint-clean.
Refs rustfs/backlog#1844 (PR2 of 3).
