docs(vehicle): revert the axis fix; the reverse is in the Chaos solver

The axis_forward:X / Blender-Z axle fix changed the reference pose but not the drive
(-2737 cm/s), and axis_forward:X also alters the shared character preset. Revert it.
The Chaos solver drives the wheel about the vehicle's Y (not the bone), and the
drive force is positive yet the car moves backward - the inversion is in the engine
torque / drive-force sign. Next: GetEngineTorque() sign or a SetupVehicle override.

Tools/blender/fbx_export_preset.py

@@ -60,7 +60,6 @@ _METRIC = {
 _CHARACTER = dict(
     _METRIC,
     object_types={"ARMATURE", "MESH"},
-    axis_forward="X",
     add_leaf_bones=False,
     path_mode="COPY",
     embed_textures=False,