The achievement timestamps were added after a few users expressed confusion that they had "randomly" gotten a new achievement having done nothing to trigger it. (Whenever a new achievement can be reliably, retroactively distributed to those who have already met the conditions, I do so.) So the timestamp was meant to indicate that the achievement is brand new and that's why they suddenly received it in their inbox despite having already met the requirements prior to that date.
Rather than remove it, since it does serve a purpose that addresses a previous problem, maybe it needs to be rephrased to something like "Achievement added to game on [timestamp]"?