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: Ifcaf6993517d02bf54cd144efd247832947a009f
(cherry picked from commit 7b649af0c1)
(cherry picked from commit 21df7f5a32)

.zuul.yaml

@@ -2,7 +2,6 @@
     templates:
       - check-requirements
       - lib-forward-testing-python3
-      - openstack-lower-constraints-jobs
       - openstack-python3-victoria-jobs
       - periodic-stable-jobs
       - publish-openstack-docs-pti

lower-constraints.txt

@@ -1,50 +0,0 @@
-alabaster==0.7.10
-appdirs==1.3.0
-Babel==2.3.4
-bandit==1.4.0
-coverage==4.0
-debtcollector==1.2.0
-docutils==0.11
-dulwich==0.15.0
-extras==1.0.0
-fixtures==3.0.0
-gitdb==0.6.4
-GitPython==1.0.1
-imagesize==0.7.1
-iso8601==0.1.11
-Jinja2==2.10
-keystoneauth1==3.4.0
-linecache2==1.0.0
-MarkupSafe==1.1.1
-mox3==0.20.0
-msgpack-python==0.4.0
-netaddr==0.7.18
-netifaces==0.10.4
-os-client-config==1.28.0
-oslo.config==5.2.0
-oslo.context==2.22.0
-oslo.i18n==3.15.3
-oslo.serialization==2.18.0
-oslo.utils==3.40.0
-oslotest==3.2.0
-pbr==2.0.0
-Pygments==2.2.0
-pyparsing==2.1.0
-python-mimeparse==1.6.0
-python-subunit==1.0.0
-pytz==2013.6
-PyYAML==5.1
-requests==2.14.2
-requests-mock==1.2.0
-requestsexceptions==1.2.0
-rfc3986==0.3.1
-six==1.10.0
-stestr==2.0.0
-smmap==0.9.0
-snowballstemmer==1.2.1
-stevedore==1.20.0
-testrepository==0.0.18
-testtools==2.2.0
-traceback2==1.4.0
-unittest2==1.1.0
-wrapt==1.7.0

tox.ini

@@ -60,10 +60,3 @@ whitelist_externals =
 commands =
   rm -rf releasenotes/build
   sphinx-build -a -E -W -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html
-
-[testenv:lower-constraints]
-deps =
-  -c{toxinidir}/lower-constraints.txt
-  -r{toxinidir}/test-requirements.txt
-  -r{toxinidir}/requirements.txt
-  -r{toxinidir}/doc/requirements.txt