remove flag: insecure-experimental-approve-all-kubelet-csrs-for-group

[Pingan2017]

What this PR does / why we need it:
this flag is deprecated in 1.7, remove it.
/kind cleanup

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

Release note:

Remove kube-controller-manager flag '--insecure-experimental-approve-all-kubelet-csrs-for-group'(deprecated in v1.7)

[Pingan2017]

/assign @deads2k

[wgliang]

/ok-to-test

[Pingan2017]

/test pull-kubernetes-integration
/test pull-kubernetes-e2e-kops-aws

[Pingan2017]

ping @kubernetes/sig-api-machinery-pr-reviews
Could someone take a look? Thanks!

[k8s-ci-robot]

@Pingan2017: Reiterating the mentions to trigger a notification:
@kubernetes/sig-api-machinery-pr-reviews

In response to this:

ping @kubernetes/sig-api-machinery-pr-reviews
Could someone take a look? Thanks!

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[Pingan2017]

ping @sttts @deads2k Could take a look? Thank u!

[Pingan2017]

Also cc @lavalamp @ingvagabund

[liggitt]

/lgtm
/approve

Please add a release note description

[Pingan2017]

ping @deads2k Could you take a look? Thank u

[liggitt]

/assign @mikedanese
for approval

[mikedanese]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: liggitt, mikedanese, Pingan2017

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• cmd/kube-controller-manager/OWNERS [mikedanese]

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