The prophecy is unfulfilled but the valonqar was never a part of the prophecy in the show. In the show Cersei is only meant to be cast down by someone younger and more beautiful... depending on how you interpret the show's events will lead you to either think it was fulfilled or wasn't.
MAGGY: Three questions you get. You won’t like the answers.
CERSEI: I’ve been promised to the prince. When will we marry?
MAGGY: You’ll never wed the prince. You’ll wed the king.
CERSEI: But I will be queen?
MAGGY: Oh yes, you’ll be queen. For a time. In comes another. Younger, more beautiful, to cast you down and take all you hold dear.
CERSEI: Will the king and I have children?
MAGGY: No. The king will have 20 children, and you will have 3.
CERSEI: That doesn’t make sense.
MAGGY: Gold will be their crowns. Gold their shrouds.
Game of Thrones, Season 5 Episode 5, "The Wars to Come"
In the books Maggy goes further to talk about the valonqar but as you can see above this is skipped in the show.
The old woman was not done with her, however. "Gold shall be their crowns and gold their shrouds," she said. "And when your tears have drowned you, the valonqar shall wrap his hands about your pale white throat and choke the life from you."
A Feast for Crows, Cersei VIII