Drop lower-constraints

Per a general OpenStack resolution [1], lower-constraints
are not worth the extra work and confusion they introduce.
This patch drops them along with all mentions.

[1] http://lists.openstack.org/pipermail/openstack-discuss/2020-December/019521.html

Change-Id: I264911c5364d2af6acef3797c011913718d2f96d

.zuul.yaml

@@ -1,7 +1,6 @@
 - project:
     templates:
       - check-requirements
-      - openstack-lower-constraints-jobs
       - openstack-python3-wallaby-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3

lower-constraints.txt

@@ -1,65 +0,0 @@
-alabaster==0.7.10
-appdirs==1.3.0
-Babel==2.3.4
-cliff==2.8.0
-cmd2==0.8.0
-coverage==4.0
-ddt==1.0.1
-debtcollector==1.2.0
-decorator==4.1.0
-deprecation==1.0
-docutils==0.11
-dogpile.cache==0.9.0
-dulwich==0.15.0
-extras==1.0.0
-fixtures==3.0.0
-imagesize==0.7.1
-iso8601==0.1.11
-Jinja2==2.10
-jmespath==0.9.0
-jsonpatch==1.16
-jsonpointer==1.13
-keystoneauth1==3.4.0
-linecache2==1.0.0
-MarkupSafe==1.1.1
-mccabe==0.2.1
-monotonic==0.6
-mox3==0.20.0
-msgpack-python==0.4.0
-munch==2.1.0
-netaddr==0.7.18
-netifaces==0.10.4
-openstackdocstheme==2.2.1
-openstacksdk==0.13.0
-os-client-config==1.28.0
-os-service-types==1.2.0
-osc-lib==1.8.0
-oslo.i18n==3.15.3
-oslo.serialization==2.18.0
-oslo.utils==3.33.0
-oslotest==3.2.0
-pbr==2.0.0
-prettytable==0.7.2
-Pygments==2.2.0
-pyparsing==2.1.0
-pyperclip==1.5.27
-python-mimeparse==1.6.0
-python-subunit==1.0.0
-pytz==2013.6
-PyYAML==3.13
-reno==3.1.0
-requests-mock==1.2.0
-requests==2.14.2
-requestsexceptions==1.2.0
-simplejson==3.5.1
-snowballstemmer==1.2.1
-Sphinx==2.0.0
-sphinxcontrib-websupport==1.0.1
-stestr==1.0.0
-stevedore==1.20.0
-testrepository==0.0.18
-testscenarios==0.4
-testtools==2.2.0
-traceback2==1.4.0
-unittest2==1.1.0
-wrapt==1.7.0

tox.ini

@@ -48,9 +48,3 @@ show-source = True
 ignore = E123,E125,W504
 builtins = _
 exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build
-
-[testenv:lower-constraints]
-deps =
-  -c{toxinidir}/lower-constraints.txt
-  -r{toxinidir}/test-requirements.txt
-  -r{toxinidir}/requirements.txt