[zuul] telemetry-tempest-plugin job: remove external_plugin override

The value of external plugin in teh variant job will be the same as in the non-variant job.
The only difference is that we always expect the varient to have a telemetry-tempest-plugin patch available

zuul.d/projects.yaml

@@ -56,10 +56,6 @@
                 override-checkout: main
             pre-run:
               - ci/debug-get-branch-for-telemetry-tempest-plugin.yml
-            vars:
-              external_plugin: "opendev.org/openstack/telemetry-tempest-plugin"
-              change_item: "{{ zuul['items'] | selectattr('project.canonical_name', 'equalto', external_plugin) }}"
-              cifmw_test_operator_tempest_external_plugin: "{{ [] if change_item | length < 1 else [ { 'repository': 'https://' + external_plugin + '.git', 'changeRepository': 'https://review' + {{ external_plugin }}, 'changeRefspec': [ 'refs/changes', change_item[0].change[-2:], change_item[0].change, change_item[0].patchset ] | join('/') } ] }}"
 
 - project:
     name: openstack-k8s-operators/telemetry-operator