remove unneeded code in playcampaign.cpp

the carryover code now reads the "next_underlying_unit_id" and [end_level_data] directly form the last snapshot which is expicitly created before the playsingle_controller object is destroyed (in playcampaign.cpp), so no reason to write them here.

also the state_of_game.carryover_sides_start["next_scenario"] = resources::gamedata->next_scenario(); is not needed since show_carryover_message doesn't do this neigher (it's done done by convert to start save later).