Drop compatibility for etcd_kubeadm_enabled

This has been deprecated for a long time, time to pull the plug.
We leave an assert for one release to have a straightforward failure if
some users were still using the variable.

roles/kubespray-defaults/tasks/main.yaml

@@ -23,12 +23,3 @@
       when:
         - http_proxy is defined or https_proxy is defined
         - no_proxy is not defined
-
-# TODO: Clean this task up when we drop backward compatibility support for `etcd_kubeadm_enabled`
-- name: Set `etcd_deployment_type` to "kubeadm" if `etcd_kubeadm_enabled` is true
-  set_fact:
-    etcd_deployment_type: kubeadm
-  when:
-    - etcd_kubeadm_enabled is defined and etcd_kubeadm_enabled
-  tags:
-    - always