Remove experimental job on openSUSE 42.3

openSUSE 42.3 is dead, remove the experimental job so
that openSUSE 42.3 can be removed completely from CI.

Note that on master this job was replaced with a newer job, but I don't
want to add a test with newer OS here.

Change-Id: I12c274c6bf88e25d9ebd4f11500b934a0ec41157

.zuul.yaml

@@ -534,22 +534,6 @@
               - ^tests-py3.txt$
               - ^tools/.*$
               - ^tox.ini$
-        - legacy-tempest-dsvm-neutron-full-opensuse-423:
-            irrelevant-files:
-              - ^(placement-)?api-.*$
-              - ^(test-|)requirements.txt$
-              - ^.*\.rst$
-              - ^.git.*$
-              - ^doc/.*$
-              - ^nova/hacking/.*$
-              - ^nova/locale/.*$
-              - ^nova/tests/.*$
-              - ^releasenotes/.*$
-              - ^setup.cfg$
-              - ^tests-py3.txt$
-              - ^tools/.*$
-              - ^tox.ini$
-
     post:
       jobs:
         - openstack-tox-cover