Remove experimental openSUSE 42.3 job

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

On master this job has been converted to 15, let's not do this on the
branch.

Change-Id: I9dd301b5e03b0d40453acbcf3a8c2d7e2232d026

.zuul.yaml

@@ -273,15 +273,6 @@
               - ^releasenotes/.*$
               - ^setup.cfg$
               - ^tox.ini$
-        - legacy-tempest-dsvm-neutron-full-opensuse-423:
-            irrelevant-files:
-              - ^(test-|)requirements.txt$
-              - ^.*\.rst$
-              - ^api-ref/.*$
-              - ^doc/.*$
-              - ^releasenotes/.*$
-              - ^setup.cfg$
-              - ^tox.ini$
     periodic:
       jobs:
         # NOTE(rosmaita): we only want the "tips" jobs to be run against