SITL Plane download and run broken, 404 in ardupilot repo

SITL download is broken, as default `plane.parm` file for an airplane is not found:

```
INFO MissionPlanner.Utilities.Download - Get https://raw.githubusercontent.com/ArduPilot/ardupilot/master/Tools/autotest/default_params/plane.parm
INFO MissionPlanner.Utilities.Download - https://raw.githubusercontent.com/ArduPilot/ardupilot/master/Tools/autotest/default_params/plane.parm NotFound
INFO MissionPlanner.GCSViews.SITL - sitl: C:\Users\kolin\Documents\Mission Planner\sitl\plane5\ C:\Users\kolin\Documents\Mission Planner\sitl\ArduPlane.exe --defaults "C:\Users\kolin\AppData\Local\Temp\tmp3525.tmp,identity.parm" -P SERIAL0_PROTOCOL=2 -P SERIAL1_PROTOCOL=2 -Mplane -s1 --home 55.0431537668193,15.17179315451,399.191083214609,90 --instance 4 --serial0 tcp:0 -P SYSID_THISMAV=5
```

Problem is, that file `C:\Users\kolin\AppData\Local\Temp\tmp3525.tmp` is empty and parameters are then broken - no simulated flying.

File `/Tools/autotest/default_params/plane.parm` is no longer there in ardupuilot codebase. And if I select `plane-3d` or `plane-jet` in drop-down menu, nothing is changed.

Start by tracing the SITL download and parameter-selection path that produces the logged plane.parm URL and temporary file, then compare the plane, plane-3d, and plane-jet selections. Done means the selected airplane configuration obtains valid default parameters and SITL starts with simulated flight instead of an empty defaults file.

Written by the indexing model from the issue text.