No bug: Disable periodic updates on esr78. r=releng-reviewers,jcristau DONTBUILD

Differential Revision: https://phabricator.services.mozilla.com/D127433

.cron.yml

@@ -241,7 +241,6 @@ jobs:
           - mozilla-central
           - mozilla-beta
           - mozilla-release
-          - mozilla-esr78
           - mozilla-esr91
       when:
           - {weekday: 'Monday', hour: 8, minute: 0}