Revert "Config: declare support for Shib IdP 2.4.x"

This reverts commit 9e5f781845253fb21580cce3a193a50258d21171.
Config: really declare support for Shib IdP 2.4.x

Now really declare support for Shib IdP 2.4.x as part of Shib 2.3.x IdP
to avoid confusion when users are looking specifically for 2.4.x.