Drop lower-constraints job

Running a lower-constraints job is optional in master, and seems
pointless in the stable branches, so drop it.

Additional information here:
http://lists.openstack.org/pipermail/openstack-discuss/2021-March/021204.html

Change-Id: I5faa237047c186e4f8bc85925040d906a1003aaa

.zuul.yaml

@@ -29,7 +29,6 @@
       - lib-forward-testing
       - lib-forward-testing-python3
       - openstack-cover-jobs
-      - openstack-lower-constraints-jobs
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs