Drop lower-constraints job

The job is not supported anymore, and failing
with errors related to SSL symbols.

Change-Id: Ia879c9b18d5a5e8d5d784323c1fc33be022a8181

.zuul.yaml

@@ -1,12 +1,10 @@
 - project:
     templates:
-      - openstack-lower-constraints-jobs
       - openstack-cover-jobs
     check:
       jobs:
         - openstack-tox-pep8
         - openstack-tox-py38
-        - openstack-tox-lower-constraints
     gate:
       jobs:
         - openstack-tox-pep8

lower-constraints.txt

@@ -1,26 +0,0 @@
-bandit==1.1.0
-coverage==4.0
-decorator==4.4.1
-eventlet==0.24.1
-fixtures==3.0.0
-flake8-import-order==0.18.1
-hacking>=3.0.1 # Apache-2.0
-netaddr==0.7.18
-openstackdocstheme==1.18.1
-oslo.i18n==3.20.0
-oslo.log==5.0.0
-oslo.serialization==2.28.1
-oslo.service==1.31.0
-oslo.utils==4.4.0
-oslotest==3.2.0
-pbr==4.0.0
-pyOpenSSL==21.0.0
-python-subunit==1.0.0
-requests-mock==1.2.0
-sphinx==2.0.0
-sphinx_feature_classification==1.0.0
-stestr==1.0.0
-tenacity==6.0.0
-testresources==2.0.0
-testtools==2.2.0
-testscenarios==0.4