Drop lower-constraints job

Lower-constraints is not a requirement of the OpenStack Python PTI
[0] and there currently is a discussion on the mailing list [1]
about dropping the test, with the oslo team already having done
so [2].

The new dependency resolver in pip fails due to incompatible
dependency versions in our lower-constraints file, meaning that
we were never providing any real guarantees with it.

To unblock the CI, I am disabling lower-constraints job for now,
with the option to reenable it in case we fix the constraints,
and based on the outcome of the mailing list discussions and
consensus.

[0]. https://governance.openstack.org/tc/reference/pti/python.html
[1]. http://lists.openstack.org/pipermail/openstack-discuss/2021-January/019672.html
[2]. http://lists.openstack.org/pipermail/openstack-discuss/2021-January/019659.html

Change-Id: I47e747058891596e7717848a0b0bc10f0a235b2f
(cherry picked from commit d6610594d1)
(cherry picked from commit 42faeb277b)
(cherry picked from commit 62c3d8eeb6)

.zuul.yaml

@@ -176,7 +176,6 @@
 - project:
     templates:
       - openstack-cover-jobs
-      - openstack-lower-constraints-jobs
       - openstack-python-jobs
       - openstack-python3-train-jobs
       - publish-openstack-docs-pti