CI - remove unnecessary logic when building images for upgrade

Docker registry being insecure is handled by docker_registry_insecure
which is set to true by default when docker_registry is set.
The removed code had no effect because docker_registry is not changed
anyway for base (pre-upgrade) install.

This change makes config more readable and also prevents a potential
conflict with the zun profile if ever used in upgrade mode.

Change-Id: I9b5ae8c5b534fa6cce9dbaca8af191e2ca79d19f
Signed-off-by: Radosław Piliszek <radoslaw.piliszek@gmail.com>

tests/templates/globals-default.j2

@@ -24,11 +24,6 @@ openstack_release: "{{ build_image_tag }}"
 {% else %}
 # use docker hub images
 docker_namespace: "kolla"
-{% if need_build_image and is_previous_release %}
-# NOTE(mgoddard): Ensure that the insecure local registry is trusted, since it
-# will be the source of images during the upgrade.
-docker_custom_option: "--insecure-registry {{ api_interface_address }}:4000"
-{% endif %}
 {% if not is_previous_release %}
 openstack_release: "{{ zuul.branch | basename }}"
 {% else %}