refactor(s3-client): drop superseded per-algorithm checksum plumbing (#6700)

refactor(s3-client): remove the superseded per-algorithm checksum plumbing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).

crates/s3-client/src/transition_api.rs

@@ -19,7 +19,6 @@
 #![allow(clippy::all)]
 
 use crate::bucket_cache::BucketLocationCache;
-use crate::checksum::ChecksumMode;
 use crate::{
     api_error_response::ErrorResponse,
     api_error_response::{err_invalid_argument, http_resp_to_error_response, to_error_response},
@@ -898,7 +897,6 @@ pub struct RequestMetadata {
     pub content_md5_base64: String,
     pub content_sha256_hex: String,
     pub stream_sha256: bool,
-    pub add_crc: ChecksumMode,
     pub trailer: HeaderMap,
 }