Currently mappers can't edit the objective screen gui.
1. But let's say you create a mission where you don't want any difference in difficulty, or set it in a different way, then I think you should be able to remove the 3 difficulty selections from the objective/dificulty screen.
(Edit: Not possible yet, but objectives could be set in mission briefing and then difficulty selection screen can be skipped, See below.)
2. Also, maybe you don't want to list the objectives on an objective screen before mission start. Maybe you want to have a mission briefing, start the mission and after that give the objectives somewhere during the mission. Or maybe you want players to select difficulty inside the mission in some way. In this case there's no need for the objectives screen and you might as well skip this screen.
(Edit: Already possible with #define ENABLE_MAINMENU_DIFF_SELECT 0 in mainmenu_custom_defs.gui )