If you would simulate "being a pax" ... why should i simulate pilots boarding the plane when im the pilot in the sim ?
The OP asked to get rid of the pop-up menu, which has been ADDED to GSX because users requested it, so this is what this topic is about. Before the menu was added, there was no choice: the crew was always boarding UNLESS the airplane code wrote some custom integration that controlled the crew, like Fslabs did in P3D.
You, instead, are asking something completely different, which is getting rid of the crew entirely, which is not the subject of this thread or this request.