[cost] Remove warning message

Summary: "Please make sure this is an expected change." makes the reports longer. We already ask for feedback on whether it is expected or not at diff time.

Reviewed By: ngorogiannis

Differential Revision: D27009995

fbshipit-source-id: d1bc6e6b1

infer/src/integration/Differential.ml

@@ -272,7 +272,6 @@ module Cost = struct
         in
         let pp_extra_msg fmt () =
           if Config.developer_mode then CostItem.pp_cost_msg fmt curr_item
-          else Format.fprintf fmt "Please make sure this is an expected change."
         in
         let ui_msg =
           if is_on_ui_thread then