Kubeadm: Remove rktlet from the CRI install docs

[neolit123]

as commented by @rosti on slack

I think, that the rktlet link needs to be removed from here:
https://kubernetes.io/docs/setup/cri/ (edited)
rktlet is stuck at CRI v1alpha1, while k8s and crictl have moved to v1alpha2

as per the good-first-issue requirements:

• you need to clone the kubernetes/website repo
• send a PR against the master branch modifying this file: https://github.com/kubernetes/website/blob/master/content/en/docs/setup/cri.md

[phoenixking25]

working on this @neolit123

[neolit123]

/lifecycle active
thanks @phoenixking25