Drop lower constraints testing

Projects like oslo are dropping lower constraints
testing. There is also discussion to drop it until
we find a proper setup. For details, please follow
the ML thread. TC will have updates soon.

http://lists.openstack.org/pipermail/openstack-discuss/2021-January/019679.html

story: 2008482
task: 41523

Change-Id: Ia91451c0f93ffe4c7a555874f8f843e01b146df5
Signed-off-by: Spyros Trigazis <spyridon.trigazis@cern.ch>
(cherry picked from commit 2fa68f127f)

.zuul.yaml

@@ -2,7 +2,6 @@
     templates:
       - check-requirements
       - openstack-cover-jobs
-      - openstack-lower-constraints-jobs
       - openstack-python3-ussuri-jobs
       - publish-openstack-docs-pti
     check: