FIX: crash after deleting a city

I've run into this problem when deleting a city as the public player: the city destructor calls check_city_tiles(true), but then remove_gebaeude_from_stadt calls check_city_tiles(false), leaving an invalid city pointer for tiles with buildings on it. The easiest fix seems to be just to call check_city_tiles again at the end of the destructor (you might also want to remove the first call).