code: private static void removeMapping(String domainName, String ip) {
          synchronized (LOCK) {
              if (DOMAIN_NAME_TO_IP.containsKey(domainName))
                  DOMAIN_NAME_TO_IP.get(domainName).remove(ip);
          }
      }
docs: 移除 域名-->IP 映射
  
  @param domainName
  @param ip