The app settings offer a way to set default browser to the system default (which is what I have selected), as well as a toggle to “Ask me every time” — I have this turned off and never see the pop-up.
EDIT: also just tested turning this checkbox off. I then clicked a link in an email, got the pop-up, unchecked “ask me every time”, clicked default browser, and didn’t see the pop-up next time.
EDIT: also just tested turning this checkbox off. I then clicked a link in an email, got the pop-up, unchecked “ask me every time”, clicked default browser, and didn’t see the pop-up next time.