I was able to remove two groups from the Views query with the code above (voorgerechten & voorgerecht).
However, when I try to rename my "voorgerechten-raam" slug, to "voorgerechten", it still says the slug is already being used, so the unset did remove some, the old slug is still "locked" somewhere.
I am able to proceed with this site, but of course it would be even better if I could just revert to the original "voorgerechten" slug.
Right now our best solution is actually what our 2nd tier support provided and it seems that this is just a one off instance because as you have it your other repeatable fields are deleting correctly.