remove mapbuffer loading when starting a special game mode

in the code before that, the special map has been generated for this special game mode, but this overwrites the content of the mapbuffer with data from disk.
This lead to the situation that the map has pointers to submaps that are not stored in mapbuffer (mapbuffer was reset on load). Those actively used submaps (still referenced from map) would not be stored as mapbuffer did not know them (mapbuffer::add_submap ignores new submaps if it already knows a submap for that point)