Stop supporting xenial amphora images

OpenStack is dropping support for xenial[1].

This patch removes references to support for xenial amphora images
and stops the nightly build of xenial amphora test images.

[1] https://review.opendev.org/#/c/695114/

Change-Id: I1bc0a37f35839a5f9a95650af7c82239f6544c50

diskimage-create/README.rst

@@ -106,7 +106,7 @@ Command syntax:
             [-a i386 | **amd64** | armhf | ppc64le ]
             [-b **haproxy** ]
             [-c **~/.cache/image-create** | <cache directory> ]
-            [-d **bionic**/**7** | xenial | 8 | <other release id> ]
+            [-d **bionic**/**7** | 8 | <other release id> ]
             [-e]
             [-f]
             [-h]

diskimage-create/diskimage-create.sh

@@ -23,7 +23,7 @@ usage() {
     echo "            [-a i386 | **amd64** | armhf | ppc64le]"
     echo "            [-b **haproxy** ]"
     echo "            [-c **~/.cache/image-create** | <cache directory> ]"
-    echo "            [-d **bionic**/**7** | xenial | 8 | <other release id> ]"
+    echo "            [-d **bionic**/**7** | 8 | <other release id> ]"
     echo "            [-e]"
     echo "            [-f]"
     echo "            [-h]"

zuul.d/jobs.yaml

@@ -32,14 +32,6 @@
         - openstack/diskimage-builder
         - openstack/octavia
         - openstack/octavia-lib
-- job:
-    name: publish-openstack-octavia-amphora-image-xenial
-    parent: publish-openstack-octavia-amphora-image
-    description: |
-      Publish Ubuntu Xenial (16.04) based amphora image to tarballs.o.o.
-    vars:
-        amphora_os: ubuntu
-        amphora_os_release: xenial
 
 - job:
     name: publish-openstack-octavia-amphora-image-bionic

zuul.d/projects.yaml

@@ -69,8 +69,6 @@
         - octavia-v2-dsvm-scenario
     periodic:
       jobs:
-        - publish-openstack-octavia-amphora-image-xenial:
-            branches: ^(?!stable/.*).*$
         - publish-openstack-octavia-amphora-image-bionic:
             branches: ^(?!stable/.*).*$
         - publish-openstack-octavia-amphora-image-centos7: