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: I665af00166cfbb49b160afce0703530de03977b5

.zuul.yaml

@@ -3,7 +3,6 @@
       - check-requirements
       - lib-forward-testing
       - lib-forward-testing-python3
-      - openstack-lower-constraints-jobs
       - openstack-python-jobs
       - openstack-python36-jobs
       - openstack-python37-jobs