Remove legacy-tempest-dsvm-neutron-{ipv6only,serviceipv6}

Those legacy jobs, only used by the master branch of neutron
in the experimental queue, are not needed right now:

- legacy-tempest-dsvm-neutron-serviceipv6 has been replaced by
  the more "mainstream" tempest-ipv6-only job, which sets the same
  configuration;
- legacy-tempest-dsvm-neutron-ipv6only is not working right now,
  it hasn't received much attention and it may need some missing
  bits in devstack. If needed, it will be recreated as native job
  in the neutron repository.

Change-Id: Ia2e63af40edd26162d1be46e8ac09c132011f757

zuul.d/projects.yaml

@@ -3451,32 +3451,6 @@
       queue: integrated
     gate:
       queue: integrated
-    experimental:
-      jobs:
-        - legacy-tempest-dsvm-neutron-ipv6only:
-            branches: master
-            irrelevant-files:
-              - ^(test-|)requirements.txt$
-              - ^.*\.rst$
-              - ^doc/.*$
-              - ^neutron/locale/.*$
-              - ^neutron/tests/unit/.*$
-              - ^releasenotes/.*$
-              - ^setup.cfg$
-              - ^tools/.*$
-              - ^tox.ini$
-        - legacy-tempest-dsvm-neutron-serviceipv6:
-            branches: master
-            irrelevant-files:
-              - ^(test-|)requirements.txt$
-              - ^.*\.rst$
-              - ^doc/.*$
-              - ^neutron/locale/.*$
-              - ^neutron/tests/unit/.*$
-              - ^releasenotes/.*$
-              - ^setup.cfg$
-              - ^tools/.*$
-              - ^tox.ini$
 
 - project:
     name: openstack/neutron-dynamic-routing