[flow] Ignore the content of exact_reason in Slice_utils.mk_object

Summary:
As the effort to replace `ExactT` with EvalT-based implementation, I want to lock down all places where we construct `ExactT`. This is the final non-EvalT-like use case, and we still haven't completely figured out why it's necessary to preserve existing behavior and perf.

This diff aims to show that at least reason is not the reason this is critical, since we can ignore the reason content, since the last call site passes in a reason for `exact_reason` to be the same as the `def_reason`.

Building on this knowledge in the next diff, I will rename this param to avoid future confusion.

Changelog: [internal]

Reviewed By: gkz

Differential Revision: D56916901

fbshipit-source-id: 2c9002247282b50614962a17c42c2b460e7810b1

src/typing/slice_utils.ml

@@ -48,15 +48,9 @@ let mk_object_type
       let t = DefT (def_reason, ObjT (mk_objecttype ~reachable_targs ~flags ~call id proto)) in
       (* Wrap the final type in an `ExactT` if we have an exact flag *)
       Base.Option.value_map
-        ~f:(fun exact_reason ->
+        ~f:(fun _ ->
           if Obj_type.is_exact flags.obj_kind then
-            let exact_reason =
-              if invalidate_aliases then
-                update_desc_reason invalidate_rtype_alias exact_reason
-              else
-                exact_reason
-            in
-            (ExactT (exact_reason, t), exact_reason)
+            (ExactT (def_reason, t), def_reason)
           else
             (t, def_reason))
         ~default:(t, def_reason)