Codechange: remove the special station/vehicle code from SaveLoad

With the new SLEG_STRUCT it is much easier to embed a struct
in a struct, where the sub-struct has limitations on when it is
being used.
This makes both the code easier to read (less magic) and avoids
the SaveLoad needing to know all these things about Stations
and Vehicles.

src/saveload/saveload_internal.h

@@ -23,7 +23,6 @@ void ResetOldNames();
 void ResetOldWaypoints();
 void MoveBuoysToWaypoints();
 void MoveWaypointsToBaseStations();
-SaveLoadTable GetBaseStationDescription();
 
 void AfterLoadVehicles(bool part_of_load);
 void FixupTrainLengths();