Drop openstack-lower-constraints-jobs

As discussed in TC PTG[1] and TC resolution[2], dropping
the lower-constraints.txt file and its testing.
We will keep lower bounds in the requirements.txt file but
with a note that these are not tested lower bounds and we
try our best to keep them updated.

[1] https://etherpad.opendev.org/p/tc-zed-ptg#L326
[2] https://governance.openstack.org/tc/resolutions/20220414-drop-lower-constraints.html#proposal

Change-Id: I0a19802c82c9ea9138a870559748063dca5507e1

Author: likui

.zuul.yaml

@@ -1,7 +1,6 @@
 - project:
     templates:
       - check-requirements
-#     - openstack-lower-constraints-jobs
       - openstack-python3-jobs
       - openstackclient-plugin-jobs
       - publish-openstack-docs-pti

lower-constraints.txt

@@ -64,10 +64,3 @@ enable-extensions = H106,H203,H904
 ignore = H202,H405,H501,W504,H306
 show-source = True
 exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build,releasenotes
-
-
-[testenv:lower-constraints]
-deps =
-  -c{toxinidir}/lower-constraints.txt
-  -r{toxinidir}/test-requirements.txt
-  -r{toxinidir}/requirements.txt