Cleanup: remove BLENDER_USER_AUTOSAVE

This was only used on WIN32 when the temporary directory didn't exist.
When the check was added [0] this made some sense because it relied on
`U.tempdir` existing, since then additional checks have been added to
ensure a temporary directory can be used. Further, this fall-back
location isn't documented in the user manual.

[0]: 615db01b01

source/blender/python/intern/bpy.cc

@@ -223,7 +223,6 @@ static PyObject *bpy_user_resource(PyObject * /*self*/, PyObject *args, PyObject
       {BLENDER_USER_DATAFILES, "DATAFILES"},
       {BLENDER_USER_CONFIG, "CONFIG"},
       {BLENDER_USER_SCRIPTS, "SCRIPTS"},
-      {BLENDER_USER_AUTOSAVE, "AUTOSAVE"},
       {0, nullptr},
   };
   PyC_StringEnum type = {type_items};