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: Idd3a92da38b4ae470f1338a79fcff26f46c6d8e4

.zuul.yaml

@@ -47,7 +47,6 @@
       - check-requirements
       - lib-forward-testing-python3
       - openstack-cover-jobs
-      - openstack-lower-constraints-jobs
       - openstack-python3-ussuri-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3