[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
  tox.ini

NOTE(elod.illes): conflict is due to branch specific settings (job
template names, lower constraints changes, tox target's base python
pinning).

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

Author: Elod Illes

.zuul.yaml

@@ -579,7 +579,6 @@
       - check-requirements
       - integrated-gate-compute
       - openstack-cover-jobs
-      - openstack-lower-constraints-jobs
       - openstack-python3-xena-jobs
       - openstack-python3-xena-jobs-arm64
       - periodic-stable-jobs

lower-constraints.txt

@@ -342,11 +342,3 @@ usedevelop = False
 deps = bindep
 commands =
   bindep test
-
-[testenv:lower-constraints]
-basepython = python3.6
-usedevelop = False
-deps =
-  -c{toxinidir}/lower-constraints.txt
-  -r{toxinidir}/test-requirements.txt
-  -r{toxinidir}/requirements.txt