⚠️ drop validation schema for objectmeta

[droot]

As per structural schema KEP
https://github.com/kubernetes/enhancements/blob/master/keps/sig-api-machinery/20190425-structural-openapi.md,
dropping support objectmeta schema generation.

[k8s-ci-robot]

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[droot]

About adding marker for x-kubernetes-embedded: true, we need to update controller-tools to 1.15, so that we can refer to new CRD jsonschema definition. So that will be added in a future PR.

[mengqiy]

/lgtm