I've been struggling on this too lately 
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Here's what you need to do:
You first need to disable the use of RADIUS on any of the Edge Service Settings. E.g. for Horizon: Open the Horizon Settings on the UAG, choose "More" then scroll up again and set "Auth Methods" to "passthrough" and save.
Now you can go to the Authentication settings and disable the RADIUS option.
As long as you have RADIUS used on an edge service somewhere, you won't be able to disable RADIUS in the authentication settings.