fix: remove all unused code (#12360)

cmd/bitrot-streaming.go

@@ -31,14 +31,6 @@ import (
 	"github.com/minio/minio/pkg/ioutil"
 )
 
-type errHashMismatch struct {
-	message string
-}
-
-func (err *errHashMismatch) Error() string {
-	return err.message
-}
-
 // Calculates bitrot in chunks and writes the hash into the stream.
 type streamingBitrotWriter struct {
 	iow          io.WriteCloser