boostrap-os: use import_tasks instead of symlinks (#11508)

Working symlinks are dependant on git configuration (when using the playbook as
a git repository, which is common), precisely `git config
core.symlinks`.

While this is enabled by default, some company policies will disable it.

Instead, use import_tasks which should avoid that class of bugs.

roles/bootstrap-os/tasks/debian.yml

@@ -62,23 +62,3 @@
     - '"changed its" in bootstrap_update_apt_result.stdout'
     - '"value from" in bootstrap_update_apt_result.stdout'
   ignore_errors: true
-
-- name: Check unattended-upgrades file exist
-  stat:
-    path: /etc/apt/apt.conf.d/50unattended-upgrades
-  register: unattended_upgrades_file_stat
-  when:
-    - os_release_dict['ID'] == 'ubuntu'
-    - ubuntu_kernel_unattended_upgrades_disabled
-
-- name: Disable kernel unattended-upgrades
-  lineinfile:
-    path: "{{ unattended_upgrades_file_stat.stat.path }}"
-    insertafter: "Unattended-Upgrade::Package-Blacklist"
-    line: '"linux-";'
-    state: present
-  become: true
-  when:
-    - os_release_dict['ID'] == 'ubuntu'
-    - ubuntu_kernel_unattended_upgrades_disabled
-    - unattended_upgrades_file_stat.stat.exists