Should I throw away my pan if I accidentally melt plastic on it? I have a pan/wok that I use for toasting flatbreads. 

Yesterday, I placed it in my cupboard while it was still hot and it accidentally touched a plastic bag full of salt. The plastic immediately melted off and stuck to the pan. 

I was able to peel off the plastic after it had solidified, but am worried there may still be some "invisible" remnants left (I have no idea if that's possible). I'm not sure what plastic the bag was made out of or anything: should I discard the pan? Or is it sufficient to wash it well?
You do not need to throw out the pan

Metals tend to be impervious to absorption of much in the way of plastics (or anything else), which is part of the reason they make great cooking implements.

If the plastic is on the cooking surface and that surface was seasoned, to be absolutely sure, I would recommend that you remove the seasoning and re-season.

If you touched the outside of the pan to the bag, then anything that remains after you peeled the plastic off will burn off next time you heat the pan.