Drop lower-constraints job

Running a lower-constraints job is optional in master, and seems
pointless in the stable branches, so drop it.

Additional information here:
http://lists.openstack.org/pipermail/openstack-discuss/2021-March/021204.html

Change-Id: I80641ff79e50cb3a519c25be712a8723367786ae

.zuul.yaml

@@ -8,7 +8,6 @@
       - release-notes-jobs
     check:
       jobs:
-        - openstack-tox-lower-constraints
         - legacy-tempest-dsvm-full-ceph-plugin-src-os-brick:
             voting: false
             irrelevant-files:
@@ -20,7 +19,6 @@
               - ^setup.cfg$
     gate:
       jobs:
-        - openstack-tox-lower-constraints
         - legacy-tempest-dsvm-full-lio-src-os-brick:
             irrelevant-files:
               - ^(test-|)requirements.txt$