[find-refs] simplify sort_and_dedup
[find-refs] simplify sort_and_dedup

Summary:
using `List.dedup_and_sort` is simpler and should be cheaper than building a map and converting back to a list.

Changelog: [internal]

Reviewed By: panagosg7

Differential Revision: D31431578

fbshipit-source-id: d1c526955e4c3363f24535dc255b0426506a509e

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