Short answer is that you can't just remove that element; it is piggy backed on to the update notification. So you would also have to stop the update notification as well. Whilst Techs suggestion is one option, but it impacts on all pop-ups.
You can either disable the update notification avastUI, Settings, Updates, Details, Update Options and uncheck the 'Show notification box after automatic update' option. Or reduce the duration for that pop-up, avastUI, Settings, Popups, Update popups, reduce that to 1 or 2 seconds (enough for notification) don't try 0 as that is just ignored.
