removes Authorizer and ExternalClientSet from kubeapiserver's admission initializer.
removes Authorizer and ExternalClientSet from kubeapiserver's admission initializer.

Kubernetes-commit: 6b1f1d14148fe3962594d8f4c9ca15d1c6171819

kubernetes-nightly/sample-apiserver,kubernetes-nightly/sample-apiserver,kubernetes/sample-apiserver,kubernetes/sample-apiserver