Anyone know how to get rid of this item?

Unfortunately you can't get rid of it.

Well, you can, but not if you've not done so properly before finishing the quest.

You didn't do the quest properly. If you happen to have a save before you start the quest, follow the guide on uesp.

There's a guy you sell it too before the quest is over. If you've really finished it, then you're stuck with it. Maybe go back to a previous save and chug through again.

You can pickpocket it to one of the NPCs after completing the quest. That's the only way I know of to get rid of it if you don't sell it to Calixto.