[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.

Change-Id: I514f6b337ffefef90a0ce9ab0b4afd083caa277e

Author: Elod Illes

.zuul.yaml

@@ -622,7 +622,6 @@
       - check-requirements
       - integrated-gate-compute
       - openstack-cover-jobs
-      - openstack-lower-constraints-jobs
       - openstack-python3-yoga-jobs
       - openstack-python3-yoga-jobs-arm64
       - periodic-stable-jobs

lower-constraints.txt

@@ -389,10 +389,3 @@ deps = bindep
 extras =
 commands =
   bindep test
-
-[testenv:lower-constraints]
-usedevelop = False
-deps =
-  -c{toxinidir}/lower-constraints.txt
-  -r{toxinidir}/test-requirements.txt
-  -r{toxinidir}/requirements.txt