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: I324cd145da8469f505ae2c135ee5035ee7008ca1

.zuul.yaml

@@ -33,7 +33,6 @@
       - check-requirements
       - lib-forward-testing-python3
       - openstack-cover-jobs
-      - openstack-lower-constraints-jobs
       - openstack-python3-xena-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3

lower-constraints.txt

@@ -1,19 +0,0 @@
-coverage==5.5
-ddt==1.4.1
-doc8==0.8.1
-docutils==0.16
-fixtures==3.0.0
-keystoneauth1==4.3.1
-oslo.i18n==5.0.1
-oslo.serialization==4.1.0
-oslo.utils==4.8.0
-pbr==5.5.0
-PrettyTable==0.7.2
-reno==3.2.0
-requests==2.25.1
-requests-mock==1.2.0
-simplejson==3.5.1
-stestr==3.1.0
-stevedore==3.3.0
-tempest==26.0.0
-testtools==2.4.0

tox.ini

@@ -116,8 +116,3 @@ import-order-style = pep8
 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,doc/source/cli/details.rst
 extension=.txt,.rst,.inc
 
-[testenv:lower-constraints]
-deps =
-  -c{toxinidir}/lower-constraints.txt
-  -r{toxinidir}/test-requirements.txt
-  -r{toxinidir}/requirements.txt