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/kubernetes/preinstall/tasks/0040-verify-settings.yml

@@ -231,25 +231,14 @@
     - not disable_host_nameservers
     - dns_mode in ['coredns', 'coredns_dual']
 
-# TODO: Clean this task up when we drop backward compatibility support for `etcd_kubeadm_enabled`
-- name: Stop if etcd deployment type is not host or kubeadm when container_manager != docker and etcd_kubeadm_enabled is not defined
+# TODO: Clean this task up after 2.28 is released
+- name: Stop if etcd_kubeadm_enabled is defined
   run_once: true
-  when: etcd_kubeadm_enabled is defined
-  block:
-    - name: Warn the user if they are still using `etcd_kubeadm_enabled`
-      debug:
-        msg: >
-          "WARNING! => `etcd_kubeadm_enabled` is deprecated and will be removed in a future release.
-          You can set `etcd_deployment_type` to `kubeadm` instead of setting `etcd_kubeadm_enabled` to `true`."
-      changed_when: true
-
-    - name: Stop if `etcd_kubeadm_enabled` is defined and `etcd_deployment_type` is not `kubeadm` or `host`
-      assert:
-        that: etcd_deployment_type == 'kubeadm'
-        msg: >
-          It is not possible to use `etcd_kubeadm_enabled` when `etcd_deployment_type` is set to {{ etcd_deployment_type }}.
-          Unset the `etcd_kubeadm_enabled` variable and set `etcd_deployment_type` to desired deployment type (`host`, `kubeadm`, `docker`) instead."
-      when: etcd_kubeadm_enabled
+  assert:
+    that: etcd_kubeadm_enabled is not defined
+    msg: |
+      `etcd_kubeadm_enabled` is removed.
+       You can set `etcd_deployment_type` to `kubeadm` instead of setting `etcd_kubeadm_enabled` to `true`."
 
 - name: Stop if download_localhost is enabled but download_run_once is not
   assert: