There are apps to unroot your phone, if warranty is your concern. I had the same nagging Avast issue and the ONLY way I got it to go away was to use the Root Explorer program (for root users only), run a search on the word “avast”, and BOOM, found one APK file with that name, even though I thought I had removed everything Avast-related from my phone.

The second I checked that file and hit delete, the warning vanished. So I guess root your phone and do that. You can always unroot later.