Remove experimental openSUSE 42.3 job

The openSUSE 42.3 distribution is eol, remove this experimental job so
that the job can be removed from Zuul.

Note that master has job for newer openSUSE running.

Change-Id: Icd25b8695816f4d89a7aa00a9b9e9387866438f3

.zuul.yaml

@@ -224,13 +224,3 @@
               - ^keystone/tests/unit/.*$
               - ^releasenotes/.*$
               - ^setup.cfg$
-        - legacy-tempest-dsvm-neutron-full-opensuse-423:
-            irrelevant-files:
-              - ^(test-|)requirements.txt$
-              - ^.*\.rst$
-              - ^api-ref/.*$
-              - ^doc/.*$
-              - ^etc/.*$
-              - ^keystone/tests/unit/.*$
-              - ^releasenotes/.*$
-              - ^setup.cfg$