add gocritic/ruleguard checks back again, cleanup code. (#13665)

- remove some duplicated code
- reported a bug, separately fixed in #13664
- using strings.ReplaceAll() when needed
- using filepath.ToSlash() use when needed
- remove all non-Go style comments from the codebase

Co-authored-by: Aditya Manthramurthy <donatello@users.noreply.github.com>

cmd/bucket-listobjects-handlers.go

@@ -59,8 +59,8 @@ func validateListObjectsArgs(marker, delimiter, encodingType string, maxKeys int
 	}
 
 	if encodingType != "" {
-		// Only url encoding type is supported
-		if strings.ToLower(encodingType) != "url" {
+		// AWS S3 spec only supports 'url' encoding type
+		if !strings.EqualFold(encodingType, "url") {
 			return ErrInvalidEncodingMethod
 		}
 	}