is it useful that the isRuntime method of Router.class?

- [x] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.
- [ ] I have checked the [FAQ](https://github.com/apache/dubbo/blob/master/FAQ.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: 2.7.2 and latest master
* Operating System version: Linux
* Java version: 1.8

### Steps to reproduce this issue

I implement the `Router.java` interface, and override the `isRuntime` method, its java doc say that it only used when the providers changed or the router rule changed, but I found that `route` method is called everytime when execute a rpc request

I search the source code, and con't found where it used.

And the `isForce` method is useless, too.