Revert "Add tempest integrated storage job to run on ubuntu focal"

Testing with focal was required as part of the testing runtime
for the 2023.1/Antelope release, which is a SLURP relase [1].
Now that Antelope is released, the development moves towards
2023.2/Bobcat where the old LTS ubuntu release is not among
the required testing runtimes[2].

Moreover, this job started failing consistenly in master.

[1] https://governance.openstack.org/tc/reference/runtimes/2023.1.html
[2] https://governance.openstack.org/tc/reference/runtimes/2023.2.html

This reverts commit b3e7d363ca.

Change-Id: I080e846c34ad91c279362f16b3e9de045776fb05

.zuul.yaml

@@ -86,8 +86,6 @@
             irrelevant-files: *gate-irrelevant-files
         - openstacksdk-functional-devstack:
             irrelevant-files: *gate-irrelevant-files
-        - tempest-integrated-storage-ubuntu-focal:
-            irrelevant-files: *gate-irrelevant-files
         - cinder-tox-py311:
             voting: false
     gate:
@@ -104,8 +102,6 @@
             irrelevant-files: *gate-irrelevant-files
         - openstacksdk-functional-devstack:
             irrelevant-files: *gate-irrelevant-files
-        - tempest-integrated-storage-ubuntu-focal:
-            irrelevant-files: *gate-irrelevant-files
     experimental:
       jobs:
         - cinder-multibackend-matrix-migration:
@@ -222,12 +218,6 @@
             DEFAULT:
               cluster: ceph
 
-- job:
-    name: tempest-integrated-storage-ubuntu-focal
-    description: Job testing for tempest-integrated-storage with Ubuntu focal (20.04)
-    parent: tempest-integrated-storage
-    nodeset: openstack-single-node-focal
-
 - job:
     name: cinder-grenade-mn-sub-bak
     parent: grenade-multinode