[2020] Stuck truck.

As far as I know, there are no ways to remove vehicles like that without mods. They should despawn on their own in the unmodded game. I am not sure why a vehicle would get so stuck that the built in despawn doesn't happen, but I suggest posting to the official support forums, so the devs can have a look. It would be very handy if you can attach the save with the stuck vehicle there.

https://forum.paradoxplaza.com/forum/threads/how-to-best-report-technical-issues-and-get-help.920608/

If the reason you want to avoid mods is Steam achievements, then there are mods, which will enable them again, so you can get rid of your stuck vehicle and get back to playing.