You want as many qualified leads as possible in a group so that you have "a pond with fish to fish in" as Dr. S says 😄 If it were me, I would leave them in there and approach the group in a broader sense...as in, I would use the group for more than just challenges.
I can only speak from experience and what has worked for me in the past but I created a group and periodically did posts in it with healthy recipes, things I was up to, mindset stuff, etc. I kept it SIMPLE. Sometimes I would just post an inspiring picture or I might create a post to get people engaged that said something like,
"Hey! Looking for some new recipes to shake up your dinner routine? I've been loving this Chicken + Broccoli Casserole! What are your favorites??"
Then from there, you could run your challenge with as much frequency as you want in the group but your posts are varied and aren't limited to JUST challenge posts. There's other stuff mixed in there.
I don't think it's necessary to delete old challenge posts if the group is structured this way but if you want to delete them you do so like this:
1. Click the three dots in the upper right hand corner of the post
2. Click Delete Post