This might be difficult. Tung oil polymerizes on contact with oxygen, forming a hard layer. Many things that could remove it, will likely damage the plastic window too. Most people recommend sanding to remove Tung Oil from wood, you can't do that with a window! I had a brush which I used with Tung Oil, and I obviously didn't clean it properly, because it stiffened about one month later. I soaked it in Citrus Solvent for 4 days, and eventually the hardened Tung Oil softened enough for me to work it off the brush.
You could spot test the window with some citrus solvent. If it doesn't damage the plastic, you try soaking it for a few days, then rubbing with a very soft cloth.
Otherwise, I don't know what to suggest. Maybe @beethinking has some experience with this?