Deprecation logic does not include removal of service-level overviews

Remove service overviews (and `-index.md` files) that do not have any packages in the latest/preview monikers.

Also, are preview versions sitting atop latest versions in the legacy moniker when moved?

There could be issues with migrating of hand-written service-level overview pages as packages move through their lifecycle. It's not feasible to deterministically assess whether hand-written content is relevant to packages being deprecated at any given point in time.