a) the character plays no big part in the books anymore either, so Martin signaled them it's okay to skip it. (the question then would be: why resurrect her in the first place?).
b) they don't need her to play an Undead they'll introduce later.
or c) She's lying. I call C), that way the book readers can be all sad and then it will be a major spoiler to everyone when it actually happens.
Well, as I said to Dom above, I doubt an actress of her age and resume would blatantly lie about such a thing. I guess we'll know sooner or later when they start filming next season as they won't be able to hide her on the set. I suspect maybe they were not sure whether to use her or not and then when they finally gave the answer, that they scratched the character, she went ahead and said she was done.
-Brandon Sanderson, The Way of Kings