Remove unused DNSAddOnType constants from kubeadm/v1alpha4

[ludusrusso]

User Story

As a developer I would like to live the code to be as simple as possible. In my PR #4516 I left this piece of code from v1alpha4 that I think can be safely removed:

cluster-api/bootstrap/kubeadm/api/v1alpha4/kubeadm_types.go

Lines 157 to 166 in 9d82562

	// DNSAddOnType defines string identifying DNS add-on types.
	type DNSAddOnType string
	const (
	// CoreDNS add-on type.
	CoreDNS DNSAddOnType = "CoreDNS"
	// KubeDNS add-on type.
	KubeDNS DNSAddOnType = "kube-dns"
	)

@fabriziopandini if you are ok with this I can submit a PR!

[sbueringer]

@ludusrusso If those are just unused constants, you can go ahead and open a PR to drop them.