Drop lower-constraints job

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 job because the complexity and recurring pain needed
to maintain that now exceeds the benefits provided by this mechanismes, and
We need to drop the lower-constraints.txt file as well as the corresponding
tox target

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

Change-Id: Ib0ef1fadf4fe44f1d6af2f7d542b8af919652501
(cherry picked from commit d783b5889f)
(cherry picked from commit a1deb5b88e)

.zuul.yaml

@@ -375,7 +375,6 @@
 - project:
     templates:
       - check-requirements
-      - openstack-lower-constraints-jobs
       - openstack-python3-ussuri-jobs
       - openstacksdk-functional-tips
       - openstacksdk-tox-tips

lower-constraints.txt

@@ -1,35 +0,0 @@
-appdirs==1.3.0
-coverage==4.0
-cryptography==2.7
-ddt==1.0.1
-decorator==4.4.1
-doc8==0.8.0
-dogpile.cache==0.6.5
-fixtures==3.0.0
-importlib_metadata==1.7.0
-iso8601==0.1.11
-jmespath==0.9.0
-jsonpatch==1.16
-jsonpointer==1.13
-jsonschema==3.2.0
-keystoneauth1==3.18.0
-linecache2==1.0.0
-munch==2.1.0
-netifaces==0.10.4
-os-service-types==1.7.0
-oslo.config==6.1.0
-oslotest==3.2.0
-pbr==2.0.0
-prometheus-client==0.4.2
-Pygments==2.2.0
-python-mimeparse==1.6.0
-python-subunit==1.0.0
-PyYAML==3.13
-requests==2.18.0
-requests-mock==1.2.0
-requestsexceptions==1.2.0
-statsd==3.3.0
-stestr==1.0.0
-testscenarios==0.4
-testtools==2.2.0
-traceback2==1.4.0

tox.ini

@@ -125,8 +125,3 @@ paths = ./openstack
 [doc8]
 extensions = .rst, .yaml
 
-[testenv:lower-constraints]
-deps =
-  -c{toxinidir}/lower-constraints.txt
-  -r{toxinidir}/test-requirements.txt
-  -r{toxinidir}/requirements.txt