fix: reduce the places where users have to register for using custom plugins

[sanposhiho]

What type of PR is this?

/area simulator /kind bug

What this PR does / why we need it:

Before this PR, users had to register their custom plugin names in registeredOutOfTreeMultiPointName. This is undocumented and led to this kind of issue: https://github.com/kubernetes-sigs/kube-scheduler-simulator/issues/311.

This PR eliminates the need to register registeredOutOfTreeMultiPointName in the first place, rather than documenting the existence of registeredOutOfTreeMultiPointName.

Which issue(s) this PR fixes:

Fixes #311

Special notes for your reviewer:

/label tide/merge-method-squash

[sanposhiho]

/retest /assign @utam0k

[utam0k]

/lgtm

[utam0k]

It seems the linter job failed. Can you check it?

make[1]: Entering directory '/home/prow/go/src/sigs.k8s.io/kube-scheduler-simulator/simulator'
golangci-lint run --timeout 90m ./...
scheduler/config/plugin.go:10: File is not `gofumpt`-ed (gofumpt)
var (
    outOfTreeRegistries = runtime.Registry{
        // TODO(user): add your plugins registries here.
    }
)
make[1]: *** [Makefile:8: lint] Error 1

[sanposhiho]

@utam0k Please revisit here when have chance.

[sanposhiho]

Rebased.

[sanposhiho]

/priority next-release

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sanposhiho, utam0k

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/kubernetes-sigs/kube-scheduler-simulator/blob/master/OWNERS)~~ [sanposhiho] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[utam0k]

/lgtm