Don't run workflow on schedule anymore, only manually

It broke once in a while, so better have an administrator manually start
it and be ready to deal with it.

.github/workflows/deploy-production-hosts.yaml

@@ -1,9 +1,5 @@
 name: Maintain & Deploy Production Hosts
 on:
-  schedule:
-    # Run Wednesday and Friday (the days on which I (Julian) usually work) at
-    # 04:00.
-    - cron: "0 4 * * 3,5"
   workflow_dispatch:
 jobs:
   Run-Everything-Playbook: