Remove ValidatingWebhookConfiguration when clearing all virtual cluster stuff

[vincent-pli]

What this PR does / why we need it:

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 #

The ValidatingWebhookConfiguration not clear when remove virtual cluster stuff, since it created by vc-manager and not belong to namespace

[k8s-ci-robot]

Hi @vincent-pli. Thanks for your PR.

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[christopherhein]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: christopherhein, vincent-pli

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