[biabd][minor] `unSome` is just `Option.value_exn`
[biabd][minor] `unSome` is just `Option.value_exn`

Summary: Easier to read that way.

Reviewed By: ezgicicek

Differential Revision: D20393926

fbshipit-source-id: 455716e10

facebook/infer,facebook/infer,facebook/infer,jvillard/infer,facebook/infer,jvillard/infer,facebook/infer,jvillard/infer,jvillard/infer,jvillard/infer,facebook/infer,jvillard/infer,jvillard/infer,facebook/infer