Expire lost items at the update instead of continuously

Replace thread DeleteItems running delete_lostitems() by simple
function delete_old_lostitems(), and call it from update_main().

src/server/main.c

@@ -383,7 +383,6 @@ start_server(int flags)
 
     empth_create(player_accept, 50 * 1024, flags, "AcceptPlayers", 0);
     empth_create(player_kill_idle, 50 * 1024, flags, "KillIdle", 0);
-    empth_create(delete_lostitems, 50 * 1024, flags, "DeleteItems", 0);
 
     market_init();
     update_init();