[cleanup] simplify Types_js.unfocused_files_to_infer
[cleanup] simplify Types_js.unfocused_files_to_infer

Summary:
this api is weird: a required option param. is there a difference between `None` and `Some (FilenameSet.empty)`? (no)

just pass `FilenameSet.empty` directly.

Changelog: [internal]

Reviewed By: samwgoldman

Differential Revision: D32610454

fbshipit-source-id: 120b6e7273fa4ae10d741c95d0aee3ae05d3b080

mroch/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow