cleanup: Remove unneeded _index.md files from docs

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.

docs/services/idp/_index.md

@@ -1,16 +0,0 @@
----
-title: IDP
-weight: 20
-geekdocRepo: https://github.com/owncloud/ocis
-geekdocEditPath: edit/master/docs/services/idp
-geekdocFilePath: _index.md
-geekdocCollapseSection: true
----
-
-## Abstract
-
-This service provides an OpenID Connect provider which is the default way to authenticate in oCIS.
-
-## Table of Contents
-
-{{< toc-tree >}}