I am trying to find this out. Right now I don't think you can get rid of Health Sync.
I think I'm only getting health data into Fit from my watch because I have Fit installed on my watch4.
But even then, Fit is only getting step data and workouts. I can't see a way to get general heart rate, breathing, blood pressure, sleep data in Fit using Health Platform currently.
Check my other thread in this community for details