Revert "Switch py36 jobs to centos-8"

This has broken a lot of projects that implicitly assume an Ubuntu
environment in their test setup, e.g. Nova and Neutron. I think that
the we need reapproach this with a previous announcement on the ML and
enough time to prepare the switch.

This reverts commit 31c4a7a18e.

Change-Id: I62f6b6a82bae2bda811c98a295d7fa0c0c9ef2cb

zuul.d/jobs.yaml

@@ -150,24 +150,6 @@
     name: openstack-tox-py36
     parent: openstack-tox
     nodeset: ubuntu-bionic
-    branches: ^(stable/(stein|train|ussuri)).*$
-    timeout: 2400
-    description: |
-
-      Run unit tests for an OpenStack Python project under cPython
-      version 3.6.
-
-      Uses tox with the ``py36`` environment.
-    irrelevant-files: *common-irrelevant-files
-    vars:
-      tox_envlist: py36
-      bindep_profile: test py36
-
-- job:
-    name: openstack-tox-py36
-    parent: openstack-tox
-    nodeset: centos-8
-    branches: ^(?!stable/(stein|train|ussuri)).*$
     timeout: 2400
     description: |