[Chore] Review `docset.yml` for subs that can be removed

In my opinion, `subs` aren't as helpful if we don't use them consistently. In https://github.com/elastic/elastic-docs-vscode/pull/2, we added inline warnings when an author hard-codes the value of a sub instead of using the sub key, which provides a light-touch way to encourage consistent use of subs and weed out unnecessary subs without introducing a bunch of errors.

However, there are some subs that are likely unnecessary, like `api-examples-title: Examples`, that should be removed from `docset.yml`. We should go through repo-level `subs` in this repo (and others) and eliminate any unnecessary `subs`.

cc @theletterf @florent-leborgne