chore: OpenVPN user and password as nullable

- Username and password can be the empty string for custom provider

internal/configuration/sources/secrets/helpers.go

@@ -25,18 +25,3 @@ func readSecretFileAsStringPtr(secretPathEnvKey, defaultSecretPath string) (
 	}
 	return files.ReadFromFile(path)
 }
-
-func readSecretFileAsString(secretPathEnvKey, defaultSecretPath string) (
-	s string, err error) {
-	path := getCleanedEnv(secretPathEnvKey)
-	if path == "" {
-		path = defaultSecretPath
-	}
-	stringPtr, err := files.ReadFromFile(path)
-	if err != nil {
-		return "", err
-	} else if stringPtr == nil {
-		return "", nil
-	}
-	return *stringPtr, nil
-}