Dropping lower constraints testing

We facing errors related to the new pip resolver, this
topic was discussed on the ML and QA team proposed to
to test lower-constraints [1].

I propose to drop this test because the complexity and recurring pain needed
to maintain that now exceeds the benefits provided by this mechanismes.

[1] http://lists.openstack.org/pipermail/openstack-discuss/2020-December/019390.html

Change-Id: Ic460410f180a49a533559c7a18bcf6b7f4850f76

.zuul.yaml

@@ -4,4 +4,3 @@
       - release-notes-jobs-python3
       - openstack-python3-victoria-jobs
       - publish-openstack-docs-pti
-      - openstack-lower-constraints-jobs