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/signature-v4-parser.go

@@ -261,7 +261,7 @@ func parseSignV4(v4Auth string, region string, stype serviceType) (sv signValues
 	// Replace all spaced strings, some clients can send spaced
 	// parameters and some won't. So we pro-actively remove any spaces
 	// to make parsing easier.
-	v4Auth = strings.Replace(v4Auth, " ", "", -1)
+	v4Auth = strings.ReplaceAll(v4Auth, " ", "")
 	if v4Auth == "" {
 		return sv, ErrAuthHeaderEmpty
 	}