Remove openSUSE Leap 42.3 jobs

openSUSE Leap 42.3 is EOL since June 2019, let's remove remaining jobs
so that we can remove Leap 42.3 from Opendev infrastructure.

Change-Id: I8c41325189f7b9e7a90d51d014bdbfc7b58371e8

.zuul.yaml

@@ -18,16 +18,6 @@
         nodes:
           - controller
 
-- nodeset:
-    name: devstack-single-node-opensuse-423
-    nodes:
-      - name: controller
-        label: opensuse-423
-    groups:
-      - name: tempest
-        nodes:
-          - controller
-
 - nodeset:
     name: devstack-single-node-opensuse-tumbleweed
     nodes:
@@ -387,13 +377,6 @@
     nodeset: devstack-single-node-centos-7
     voting: false
 
-- job:
-    name: devstack-platform-opensuse-423
-    parent: tempest-full
-    description: openSUSE 43.2 platform test
-    nodeset: devstack-single-node-opensuse-423
-    voting: false
-
 - job:
     name: devstack-platform-opensuse-tumbleweed
     parent: tempest-full
@@ -481,7 +464,6 @@
       jobs:
         - devstack
         - devstack-platform-centos-7
-        - devstack-platform-opensuse-423
         - devstack-platform-opensuse-tumbleweed
         - devstack-platform-fedora-latest
         - devstack-multinode