Dropping lower constraints testing (stable Xena)

This topic was discussed on the ML and QA team proposed to
to test lower-constraints [1].

As decided in Neutron meeting, stable branches will drop this
CI job [2].

[1]http://lists.openstack.org/pipermail/openstack-discuss/2020-December/019390.html
[2]http://eavesdrop.openstack.org/meetings/networking/2021/networking.2021-01-12-14.00.log.html

Change-Id: I9559c5810d0dfff0793cb3094e007a9cb8f681c6

zuul.d/project.yaml

@@ -6,7 +6,6 @@
     templates:
       - neutron-tempest-plugin-jobs
       - openstack-cover-jobs
-      - openstack-lower-constraints-jobs
       - openstack-python3-xena-jobs
       - openstack-python3-xena-jobs-arm64
       - publish-openstack-docs-pti
@@ -47,8 +46,6 @@
             timeout: 3600
         - openstack-tox-py38:  # from openstack-python3-xena-jobs template
             timeout: 3600
-        - openstack-tox-lower-constraints:  # from openstack-tox-lower-constraints template
-            timeout: 3600
         - openstack-tox-cover:  # from openstack-cover-jobs template
             timeout: 4800
         - neutron-ovn-tempest-slow
@@ -68,8 +65,6 @@
         - neutron-ovn-tempest-ovs-release-ipv6-only
         - openstack-tox-py36:  # from openstack-python3-xena-jobs template
             timeout: 3600
-        - openstack-tox-lower-constraints:  # from openstack-tox-lower-constraints template
-            timeout: 3600
         - neutron-ovn-tempest-slow
         #- neutron-ovn-rally-task
         #- neutron-ovn-tripleo-ci-centos-8-containers-multinode