Drop lower constraints testing

It's no longer required, and there doesn't seem to be any benefit,
so remove the lower constraints file, job, and tox testenv.

Change-Id: I1c62a637050eb8ee664edbe8f0fcb2e6cd98cec7

.zuul.yaml

@@ -1,6 +1,5 @@
 - project:
     templates:
-      - openstack-lower-constraints-jobs
       - openstack-python3-xena-jobs
       - publish-openstack-docs-pti
       - check-requirements
@@ -16,7 +15,6 @@
     timeout: 2400
     irrelevant-files:
       - ^(test-|)requirements.txt$
-      - ^lower-constraints.txt$
       - ^.*\.rst$
       - ^rbd_iscsi_client/locale/.*$
       - ^doc/.*$

lower-constraints.txt

@@ -1,17 +0,0 @@
-Babel==2.3.4
-coverage==4.1
-doc8==0.6.0
-ddt==1.2.1
-hacking==3.1.0
-openstackdocstheme==2.2.7
-oslo.i18n==3.24.0
-oslo.log==3.44.0
-oslo.utils==3.36.0
-oslotest==3.2.0
-pbr==5.4.1
-reno==3.1.0
-requests==2.14.2
-sphinx==2.0.0
-stestr==1.0.0
-testscenarios==0.4
-testtools==2.2.0

tox.ini

@@ -130,9 +130,3 @@ import_exceptions = rbd_iscsi_client.i18n
 deps = bindep
 commands = bindep test
 usedevelop = False
-
-[testenv:lower-constraints]
-deps =
-  -c{toxinidir}/lower-constraints.txt
-  -r{toxinidir}/test-requirements.txt
-  -r{toxinidir}/requirements.txt