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/object-api-utils.go

@@ -790,7 +790,7 @@ func (g *GetObjectReader) Close() error {
 	return nil
 }
 
-//SealMD5CurrFn seals md5sum with object encryption key and returns sealed
+// SealMD5CurrFn seals md5sum with object encryption key and returns sealed
 // md5sum
 type SealMD5CurrFn func([]byte) []byte