ci: Only install default Python 3.x during image prep

Newer images will shortly be generated, so these higher Python versions aren't
needed anymore.

tests/image_prep/_container_setup.yml

@@ -27,12 +27,6 @@
     - role: sshd_container
 
   tasks:
-    - name: Configure /usr/bin/python
-      command: alternatives --set python /usr/bin/python3.8
-      args:
-        creates: /usr/bin/python
-      when: inventory_hostname in ["centos8"]
-
     - name: Enable UTF-8 locale on Debian
       copy:
         dest: /etc/locale.gen

tests/image_prep/host_vars/centos8.yml

@@ -6,8 +6,6 @@ packages:
   - perl-JSON
   - python2-virtualenv
   - python3-virtualenv
-  - python36
-  - python38
 
 package_manager_repos:
   - dest: /etc/yum.repos.d/CentOS-Linux-AppStream.repo