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>

internal/kms/single-key.go

@@ -191,7 +191,7 @@ func (kms secretKey) DecryptKey(keyID string, ciphertext []byte, context Context
 		mac.Write(encryptedKey.IV)
 		sealingKey := mac.Sum(nil)
 
-		block, err := aes.NewCipher(sealingKey[:])
+		block, err := aes.NewCipher(sealingKey)
 		if err != nil {
 			return nil, err
 		}