ubuntu-systemd-container: deprecate and remove jobs

I'm not aware this element is used/was ever used.  It hasn't ever been
updated to Focal.  To reduce our testing footprint remove this test,
and note in the element its probably broken.

Change-Id: I17cd3b13948287fe78990cfbe16a22919a329ba9

.zuul.d/jobs.yaml

@@ -66,19 +66,6 @@
         - gentoo/build-succeeds
         - debian-minimal/stable-vm
 
-- job:
-    name: dib-functests-containers
-    description: |
-      These are tests for elements focused on containers or other
-      small deployments
-    abstract: true
-    voting: false
-    parent: dib-functests-base
-    vars:
-      dib_functests:
-        - ubuntu-systemd-container/bionic-build-succeeds
-        - ubuntu-systemd-container/xenial-build-succeeds
-
 - job:
     name: dib-functests-arm64
     description: |
@@ -116,13 +103,6 @@
     nodeset: ubuntu-bionic
     voting: false
 
-- job:
-    name: dib-functests-bionic-python3-containers
-    description: |
-      The containers tests on a Bionic build host with Python 3
-    parent: dib-functests-containers
-    nodeset: ubuntu-bionic
-    voting: false
 
 # ARM64 tests
 - job:

.zuul.d/project.yaml

@@ -29,7 +29,6 @@
         - dib-functests-bionic-python3
         - dib-functests-bionic-python3-image
         - dib-functests-bionic-python3-extras
-        - dib-functests-bionic-python3-containers
         - ironic-python-agent-check-image-dib-centos8
         - dib-nodepool-functional-openstack-debian-bullseye-src:
             voting: false

diskimage_builder/elements/ubuntu-systemd-container/README.rst

@@ -2,6 +2,10 @@
 ubuntu-systemd-container
 ========================
 
+.. note::
+
+   As of October 2021 this element is deprecated and unmaintained.
+
 The ``ubuntu-systemd-container`` element uses debootstrap for generating
 a minimal image for use by machine containers. In contrast the ``ubuntu``
 element uses the cloud-image as the initial base and the ``ubuntu-minimal``