Drop link to CDK on LXD

The page this linked to recommends considering microk8s, so let's omit
this one and leave microk8s.

Why leave microk8s? It's a certified Kubernetes distribution focused on
learning environments, and it's multiplatform (ish).

Author: Tim Bannister

content/en/docs/setup/_index.md

@@ -40,9 +40,8 @@ If you're learning Kubernetes, use the Docker-based solutions: tools supported b
 
 |Community           |Ecosystem     |
 | ------------       | --------     |
-| [Minikube](/docs/setup/learning-environment/minikube/) | [CDK on LXD](https://www.ubuntu.com/kubernetes/docs/install-local) |
-| [kind (Kubernetes IN Docker)](/docs/setup/learning-environment/kind/) | [Docker Desktop](https://www.docker.com/products/docker-desktop)|
-|                     | [Minishift](https://docs.okd.io/latest/minishift/)|
+| [Minikube](/docs/setup/learning-environment/minikube/) | [Docker Desktop](https://www.docker.com/products/docker-desktop)|
+| [kind (Kubernetes IN Docker)](/docs/setup/learning-environment/kind/) | [Minishift](https://docs.okd.io/latest/minishift/)|
 |                     | [MicroK8s](https://microk8s.io/)|