process-updates-* already calls bincraft::upload_package_index() at
the end of each daily run, and PR #71 added the CDN purge there too.
The 14-step update-package-index-* chain duplicated that work
(~40 min cold rebuild per platform) and serialized purely to dodge
S3 rate limits, even though no one was waiting for the index after
process-updates had landed it.
Deletes the chain. The 'update-package-index' and 'update-package-index-*'
cron names referenced inside those files become inert with the YAMLs;
the matching cron entries on the Crow side can be removed in a follow-up.