Missleading '[email protected]` property on group and educationClass

[rhafer]

Our definition of the group and educationClass resources contain a somewhat missleading [email protected] property (see e.g. https://github.com/owncloud/libre-graph-api/blob/main/api/openapi-spec/v1.0.yaml#L2245).

This is not really a property and cause confusion when showing up that way in the swagger ui. The @odata.bind is actually just an annotation that can be used in POST/PATCH request to manage relations between resources.

I am not sure yet whether our tooling allows to specify this in a better manner, but we should investigate this to avoid confusion.