remove useless registry backend in nydusify about nydus HOT 2 CLOSED

func getBackendConfig(c *cli.Context, suffix string, required bool) (string, string, error) {
	backendType := c.String(suffix + "backend-type")
	if backendType == "" {
		if required {
			return "", "", errors.Errorf("backend type is empty, please specify option '--%sbackend-type'", suffix)
		}
		return "", "", nil
	}

	possibleBackendTypes := []string{"oss", "s3"}
	if !isPossibleValue(possibleBackendTypes, backendType) {
		return "", "", fmt.Errorf("--%sbackend-type should be one of %v", suffix, possibleBackendTypes)
	}
// ...
}