Drop lower-constraints.txt and its testing

As discussed in TC PTG[1] and TC resolution[2], we are
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: Ifbc383f6d4c858189cee55e67b4d4274d9c3358a

tox.ini

@@ -391,10 +391,3 @@ deps = bindep
 extras =
 commands =
   bindep test
-
-[testenv:lower-constraints]
-usedevelop = False
-deps =
-  -c{toxinidir}/lower-constraints.txt
-  -r{toxinidir}/test-requirements.txt
-  -r{toxinidir}/requirements.txt