When I park at the jetway and call for passengers boarding, a stairway appears at the rear door (passenger door #2) of the aircraft.
If with "rear jetway", you mean passenger door #4 (which is the one in the rear, unless you reorder them using the configuration editor), that's absolutely normal, rear stairways are used quite often, even if there's a jetway, if only for the maintenance/cleaning crew.
Of course, as explained on the GSX manual, Page 22, this can be customized on such parking were the rear jetway might not fit:
Disable staircase
This option will disable all passenger stairs, even the rear one on a jetway-equipped parking.
If, instead, you want to remove it everywhere, you should remove it on the aircraft configuration editor instead.
