Retire tempest-horizon

As disscussed in Wallaby PTG [1], QA and Horizon team decided to
move the horizon dashboard test from tempest-horizon to Tempest.
Scenario tests in tempest-horizon have been merged into the tempest
repository. tempest-horizon is branchless so we can retire it
without a deprecation phase.

Depends-On: https://review.opendev.org/c/openstack/project-config/+/774378
Change-Id: Ie18ae4891d41e39d9cb23aee50f11b329e724172

reference/legacy.yaml

@@ -303,6 +303,10 @@ horizon:
       retired-on: 2018-02-07
       repos:
         - openstack/horizon-cisco-ui
+    tempest-horizon:
+      retired-on: 2021-02-08
+      repos:
+        - openstack/tempest-horizon
     xstatic-angular-ui-router:
       retired-on: 2019-03-24
       repos:

reference/projects.yaml

@@ -469,9 +469,6 @@ horizon:
         - assert:follows-standard-deprecation
         - assert:supports-upgrade
         - stable:follows-policy
-    tempest-horizon:
-      repos:
-        - openstack/tempest-horizon
     ui-cookiecutter:
       release-management: none
       repos: