How to remove this PVC to cast iron adapter?
If it's all one piece I'm inclined to believe you have a donut and coupling similar to the ones below installed in you cast iron pipe. 





I have only used these one time, but I had to hammer the piece of PVC into to rubber donut because it fit so tight. Some PVC fittings have a lip at the tip, like the one below, that would  make it harder to remove once it was installed. I think you're going to have to work it out, pulling and twisting, using a lot of elbow grease.