Delete "kubeadm alpha certs" code (#8322)

"kubeadm alpha certs" command has been promoted to "kubeadm certs" command,
and "kubeadm alpha certs" has been deprecated since Kubernetes v1.20 as [1].
In addition, Kubespray supports Kubernetes v1.20+.
This delete the deprecated command for cleanup.

[1]: https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG/CHANGELOG-1.20.md#deprecation