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: Ibd446707bbdb46f4c3fe5b4f8d7dd754d79c2db0

.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

-appdirs==1.4.3
-certifi==2018.1.18
-cffi==1.13.1
-chardet==3.0.4
-cliff==3.1.0
-cmd2==0.8.9
-debtcollector==2.1.0
-decorator==4.4.2
-dogpile.cache==0.9.2
-entrypoints==0.3
-etcd3gw==0.2.0
-extras==1.0.0
-fixtures==3.0.0
-future==0.18.2
-futurist==2.2.0
-gitdb2==2.0.3
-GitPython==2.1.8
-idna==2.6
-iso8601==0.1.12
-keystoneauth1==3.4.0
-linecache2==1.0.0
-mock==4.0.2
-mox3==0.25.0
-msgpack==1.0.0
-netaddr==0.7.19
-netifaces==0.10.9
-os-client-config==1.29.0
-oslo.config==6.9.0
-oslo.context==3.1.0
-oslo.i18n==5.0.0
-oslo.log==3.44.0
-oslo.serialization==3.2.0
-oslo.utils==3.41.0
-oslotest==3.2.0
-pbr==3.1.1
-pifpaf==0.10.0
-prettytable==0.7.2
-prometheus-client==0.6.0
-pycparser==2.18
-pyinotify==0.9.6
-pymongo==3.0.2
-pyparsing==2.2.0
-pyperclip==1.8.0
-python-dateutil==2.8.1
-python-memcached==1.56
-python-mimeparse==1.6.0
-python-subunit==1.2.0
-pytz==2020.1
-PyYAML==3.12
-requests==2.18.4
-requestsexceptions==1.4.0
-rfc3986==1.4.0
-six==1.11.0
-smmap2==2.0.3
-stestr==2.0.0
-stevedore==1.28.0
-testrepository==0.0.20
-testtools==2.3.0
-traceback2==1.4.0
-unittest2==1.1.0
-urllib3==1.22
-voluptuous==0.11.7
-wcwidth==0.2.4
-wrapt==1.12.1
-xattr==0.9.3

tox.ini

@@ -59,8 +59,3 @@ deps = {[testenv:docs]deps}
 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