[stable-only] Drop lower-constraints job

During the PTG the TC discussed the topic and decided to drop the job
completely. Since the latest job configuration broke all stable gate
for nova (older than yoga) this is needed there to unblock our gates.
For dropping the job on master let's wait to the resolution as the
gate is not broken there, hence the patch is stable-only.

Conflicts:
  .zuul.yaml
  lower-constraints.txt

NOTE(elod.illes): conflict is due to branch specific settings (job
template names, lower constraints changes).

Change-Id: I514f6b337ffefef90a0ce9ab0b4afd083caa277e
(cherry picked from commit 15b7271)
(cherry picked from commit ba3c5b8)
(cherry picked from commit 327693a)

Author: Elod Illes

.zuul.yaml

@@ -417,7 +417,6 @@
       - check-requirements
       - integrated-gate-compute
       - openstack-cover-jobs
-      - openstack-lower-constraints-jobs
       - openstack-python3-victoria-jobs
       - periodic-stable-jobs
       - publish-openstack-docs-pti

lower-constraints.txt

@@ -335,10 +335,3 @@ usedevelop = False
 deps = bindep
 commands =
   bindep test
-
-[testenv:lower-constraints]
-usedevelop = False
-deps =
-  -c{toxinidir}/lower-constraints.txt
-  -r{toxinidir}/test-requirements.txt
-  -r{toxinidir}/requirements.txt