replace PlayMusicOnMapLoad trait with Lua Media.PlayMusic method

mods/cnc/maps/shellmap/shellmap.lua

@@ -17,6 +17,7 @@ WorldLoaded = function()
 	for i, unit in ipairs(units) do
 		LoopTrack(unit, CPos.New(8, unit.Location.Y), CPos.New(87, unit.Location.Y))
 	end
+	PlayMusic()
 end
 
 LoopTrack = function(actor, left, right)
@@ -25,6 +26,10 @@ LoopTrack = function(actor, left, right)
 	actor.CallFunc(function() LoopTrack(actor, left, right) end)
 end
 
+PlayMusic = function()
+	Media.PlayMusic("map1", PlayMusic)
+end
+
 LoadTransport = function(transport, passenger)
 	transport.LoadPassenger(Actor.Create(passenger, false, { Owner = transport.Owner, Facing = transport.Facing }))
 end