This commit removes the `doc/service/<name>/_index.md` files for service
that provide a README.md in their own directory.
The CI's docs pipeline generates and publishes (to the `docs` branch)
the _index.md files from the READMEs. We don't need to have these files
duplicated `master` branch of the git repo. They just cause confusion
and are not kept in sync with the READMEs on the `master` branch.
