[hack] OCaml warnings: treat unused variables as errors everywhere
[hack] OCaml warnings: treat unused variables as errors everywhere

Summary: We want to minimise the number of places where we override errors. We have three places where we set error levels (buck, dune, and .merlin), so customising error levels elsewhere is complicated and often unnecessary.

Reviewed By: kmeht

Differential Revision: D15984166

fbshipit-source-id: 74285913d2d8f88dbbf16298f7fb2b9644a6cb23

facebook/flow,facebook/flow,gabelevi/flow,mroch/flow,samwgoldman/flow,samwgoldman/flow,facebook/flow,gabelevi/flow,mroch/flow,gabelevi/flow,mroch/flow,mroch/flow,mroch/flow,gabelevi/flow,samwgoldman/flow,facebook/flow,gabelevi/flow,facebook/flow,mroch/flow,facebook/flow,samwgoldman/flow,gabelevi/flow,samwgoldman/flow,mroch/flow,gabelevi/flow,samwgoldman/flow,samwgoldman/flow,facebook/flow