Dark Forecast: don't modify unit movement costs

Preferrably leaving them in the vanilla version.

Closes #3891

data/multiplayer/scenarios/2p_Dark_Forecast.lua

@@ -214,20 +214,6 @@ end
 local function place_units(unittypes, x, y)
 	for i,v in ipairs(unittypes) do
 		local u = wesnoth.units.create { type = v, generate_name = true, side = 2 }
-		u:add_modification("object", {
-			T.effect {
-				apply_to = "movement_costs",
-				replace = true,
-				T.movement_costs {
-					flat = 1,
-					sand = 2,
-					forest = 2,
-					impassable = 3,
-					unwalkable = 3,
-					deep_water = 3,
-				}
-			}
-		})
 		-- give the unit less moves on its first turn.
 		u.status.slowed = true
 		u:add_modification("object", {