Deleting `matterhorn-reference` users

[JulianKniephoff]

Forward-port opencast/opencast#4789 (opencast/opencast#5058).

[JulianKniephoff]

This will be resolved by opencast/opencast#5833.

[JulianKniephoff]

@Arnei brought up a valid concern with that approach. I will probably just forward port his original implementation then.