Remove unused UNUSED
Remove unused UNUSED

Summary: Missed this when cleaning up the UNUSED in D22354198 (https://github.com/facebook/fbthrift/commit/091602b3000124b7273bba33f6dea9ec63a6bd8f)

Reviewed By: yfeldblum

Differential Revision: D22363970

fbshipit-source-id: d2ec931bc5ebead32444c6ab37cda44c95f9c702

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift