These files contains all the ground vehicles around the terminals, like catering, push back, baggage carts, etc.
It's quite safe to remove them but, since these objects appears only on specific Scenery Complexity levels, you would have the same effect by simply lowering that slider, which impacts also the rest of the scenery, especially the Manhattan area, which is the real fps eater.
Note that, the biggest increase in fps is not reached by dumbing down JFK, since the scenery is already very optimized. The biggest fps impact for this airport doesn't have anything to do with scenery, and are the AI that, since the airports has lot more parking stands than the default, might come in numbers. So, by adjusting the AI settings, you should probably find the biggest effect on fps.