Drop lower-constraints

Per our PTG resolution [1] and general OpenStack resolution [2], 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-October/018445.html
[2] http://lists.openstack.org/pipermail/openstack-discuss/2020-December/019521.html

Change-Id: I58807c27fd20c7269019e26e8b507744d35325f3
(cherry picked from commit 8e9323d)

Author: yoctozepto

.zuul.d/base.yaml

@@ -74,7 +74,6 @@
       - ^.*\.rst$
       - ^doc/.*
       - ^etc/.*
-      - ^lower-constraints.txt$
       - ^releasenotes/.*$
       - ^specs/.*$
       - ^test-requirements.txt$

lower-constraints.txt

@@ -107,9 +107,3 @@ extension =
     K301 = checks:no_mutable_default_args
     K302 = checks:no_log_warn
 paths = ./kolla/hacking
-
-[testenv:lower-constraints]
-deps =
-  -c{toxinidir}/lower-constraints.txt
-  -r{toxinidir}/test-requirements.txt
-  -r{toxinidir}/requirements.txt