Clean up `ExternalSchedulerEnabled`

[sanposhiho]

We had the feature called ExternalSchedulerEnabled that disable the scheduler in the simulator and allow users to use their own scheduler. https://github.com/kubernetes-sigs/kube-scheduler-simulator/blob/master/simulator/docs/external-scheduler.md

But, after https://github.com/kubernetes-sigs/kube-scheduler-simulator/pull/370, we no longer have any use case for it.

So, we can clean up ExternalSchedulerEnabled related implementation, and also have to modify the documentation - https://github.com/kubernetes-sigs/kube-scheduler-simulator/blob/master/simulator/docs/external-scheduler.md

Note that, we cannot just remove ExternalSchedulerEnabled configuration from the config API because that'd be breaking change. We clean up all related implementation and doc internally, but we keep ExternalSchedulerEnabled. (i.e., even when users use it, it'd be no-op.)

/kind cleanup /area simulator

[sanposhiho]

/priority next-release

[sanposhiho]

@YamasouA are you interested in it by chance? Just a easy clean up.

[YamasouA]

/assign