Don't do "unload" at Sys_Shutdown time

When the engine is shutting down, there's no need to unload;
everything will simply be shut down.

Todo: Is the game plugin shut down properly without unloading?

doomsday/engine/portable/src/sys_system.c

@@ -138,10 +138,6 @@ boolean Sys_IsShuttingDown(void)
  */
 void Sys_Shutdown(void)
 {
-    // Time to unload *everything*.
-    if(DD_GameLoaded())
-        Con_Execute(CMDS_DDAY, "unload", true, false);
-
     B_Shutdown();
     Sys_ShutdownTimer();