Dropping lower constraints testing

We facing errors related to the new pip resolver, this
topic was discussed on the ML and QA team proposed to
to test lower-constraints [1].

I propose to drop this test because the complexity and recurring pain needed
to maintain that now exceeds the benefits provided by this mechanismes.

[1] http://lists.openstack.org/pipermail/openstack-discuss/2020-December/019390.html

Change-Id: I8d29322460f901b0f034d8f08d109a06f9255df4

.zuul.yaml

@@ -60,7 +60,6 @@
 - project:
     templates:
       - check-requirements
-      - openstack-lower-constraints-jobs
       - openstack-python3-xena-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3

lower-constraints.txt

@@ -1,11 +0,0 @@
-coverage==5.5
-ddt==1.4.1
-doc8==0.8.1
-os-brick==4.3.0
-oslo.concurrency==4.4.0
-oslotest==4.4.1
-pbr==5.5.1
-python-cinderclient==7.3.0
-stestr==3.1.0
-testscenarios==0.5.0
-testtools==2.4.0

tox.ini

@@ -96,8 +96,3 @@ exclude=.venv,.git,.tox,dist,doc/*,*lib/python*,*egg,build
 ignore-path=.tox,*.egg-info,doc/src/api,doc/source/drivers.rst,doc/build,.eggs/*/EGG-INFO/*.txt,doc/source/configuration/tables,./*.txt,releasenotes/build
 extension=.txt,.rst,.inc
 
-[testenv:lower-constraints]
-deps =
-  -c{toxinidir}/lower-constraints.txt
-  -r{toxinidir}/test-requirements.txt
-  -r{toxinidir}/requirements.txt