Drop lower-constraints.txt

The content has never been tested in CI. Lower constraints are no
longer tested in neutron since l-c job was removed[1].

[1] bc8e512b4a7b68f2e5c31efd320d8b347d06bba0

Change-Id: I99c7ca171638cf058ec216a29e26f522d580fdae

lower-constraints.txt

@@ -1,4 +0,0 @@
-# You need to rewrite this file with the project's lower constraints.
-# This can be done using the following script:
-#
-# https://opendev.org/openstack/requirements/src/branch/master/tools/fix-lower-constraints.py

tox.ini

@@ -18,10 +18,6 @@ deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/
        -r{toxinidir}/test-requirements.txt
 commands = stestr run --exclude-regex ".tests.functional" {posargs}
 
-[testenv:lower-constraints]
-deps = -c{toxinidir}/lower-constraints.txt
-       -r{toxinidir}/test-requirements.txt
-
 [testenv:pep8]
 commands = flake8 {posargs}