(svn r8660) -Fix

-Codechange: Remove _avail_aircraft
Its name is misleading. It is rather _avail_airports, but then only some of them, which leads to inconsistencies when using it.
Further it is unnecessary to store it in savegams.

src/variables.h

@@ -50,9 +50,6 @@ VARDEF VehicleID _vehicle_id_ctr_day;
 // Skip aging of cargo?
 VARDEF byte _age_cargo_skip_counter;
 
-// Available aircraft types
-VARDEF byte _avail_aircraft;
-
 // Position in tile loop
 VARDEF TileIndex _cur_tileloop_tile;