Remove experimental job legacy-tempest-dsvm-zeromq-multibackend

The devstack-plugin-zeromq is retired, see
https://review.opendev.org/748700. Remove this experimental job
to fix Zuul configuration.

Change-Id: I37c028ab063ec9127f02dd479cae49da8bffb11b
(cherry picked from commit 0e72ffa987)

.zuul.yaml

@@ -172,21 +172,6 @@
               - ^tox.ini$
     experimental:
       jobs:
-        - legacy-tempest-dsvm-zeromq-multibackend:
-            irrelevant-files:
-              - ^(test-|)requirements.txt$
-              - ^.*\.rst$
-              - ^api-ref/.*$
-              - ^cinder/hacking/.*$
-              - ^cinder/locale/.*$
-              - ^cinder/tests/functional.*$
-              - ^cinder/tests/unit.*$
-              - ^contrib/block-box.*$
-              - ^doc/.*$
-              - ^releasenotes/.*$
-              - ^setup.cfg$
-              - ^tools/.*$
-              - ^tox.ini$
         - legacy-tempest-dsvm-multibackend-matrix:
             irrelevant-files:
               - ^(test-|)requirements.txt$