Allergies are not in the game so there's nothing to remove. Even if they were (spoiler: they're not, this is hypothetical) you wouldn't have to choose the trait for your sims.
What is the real issue here? You're concerned about something which isn't in the game. You're concerned that Sims 4, of all games, will have sims suffering and/or dying from it. Sims 4 has a lot of death types, but they are goofy deaths for the most parts. Gone are the Sims 2 days where illness could kill sims or the Sims 3 days where you could get an allergic reaction from picking wildflowers -- in Sims 4, illness gives a temporary buff which gives you tiger stripes or something and apparently, a dairy intolerance is going to make your sim fart for a few hours, but only if you allow the sim to have lactose intolerance to begin with.
