[CI] Stop testing lower-constraints

This patch disables the lower-constraints job as it started
failing and we agreed during the PTG to drop it anyway. [1]

[1] http://lists.openstack.org/pipermail/openstack-discuss/2020-October/018445.html

Change-Id: Icfedd16ef9aed84e9f9be939e832c35e0d735efe
(cherry picked from commit bc0a41c)

Author: yoctozepto

.zuul.d/project.yaml

@@ -1,7 +1,6 @@
 ---
 - project:
     templates:
-      - openstack-lower-constraints-jobs
       - openstack-python3-ussuri-jobs
       - check-requirements
       - publish-openstack-docs-pti