What's the function/meaning of 就不要 in 过去的事情就不要说了?
过去的事情就不要说了 means it's not necessary to mention those things in the past. Or it's not deserve to mention the things in the past. It implies that just let it go because it is just meaningless/helpless to the current situation. 

就 here is kinda like the word just. So, 就不要 here is kinda like just don't because there is no use.

You should comprehend 就不要 as 就 and 不要 in this case. 不要 is like "don't" in an imperative sentence. For example, 不要说话: don't speak! 就不要说话了 is kind of suggestion, meaning just don't say anything.

Another point is 过去的事情就不要说了 can be paraphrased as 就不要说过去的事情了. By pulling 过去的事情 ahead, it's like saying: things passed, so just don't mention it.