base: drop the one-off crontab migration

The prune job now lives in /etc/cron.d on both hosts and root's crontab
entry is gone, so the tasks that removed it have nothing left to do.

Assisted-by: Claude:fable-5-2

ansible/roles/base/tasks/main.yaml

@@ -59,18 +59,3 @@
     content: |
       {{ docker_prune_schedule }} root docker image prune -af > /var/log/docker-prune.log 2>&1
     mode: "0644"
-
-- name: Check for the old prune entry in root's crontab
-  ansible.builtin.command: crontab -l
-  register: root_crontab
-  changed_when: false
-  failed_when: false
-  check_mode: false
-
-- name: Drop the old prune entry from root's crontab
-  ansible.builtin.shell: |
-    set -o pipefail
-    crontab -l | { grep -v 'docker image prune' || true; } | crontab -
-  args:
-    executable: /bin/bash
-  when: "'docker image prune' in root_crontab.stdout"