CI: Remove disable_selinux_do_reboot flag

This is not needed anymore because the flag got renamed in Zed.

Change-Id: I0187f9a3f23dc59582059d2c7eb4ca1b283002b4

playbooks/kayobe-overcloud-upgrade-base/overrides.yml.j2

 ---
-# NOTE(mgoddard): Don't reboot after disabling SELinux during CI testing, as
-# Ansible is run directly on the controller.
-# TODO(priteau): This is needed for the deployment of the previous release.
-# Remove when previous_release is zed.
-disable_selinux_do_reboot: false
-
 # Use the OpenStack infra's Dockerhub mirror.
 docker_registry_mirrors:
   - "http://{{ zuul_site_mirror_fqdn }}:8082/"