[nullsafe] Remove uneccessary logic in chosing join priority

Summary:
This was perhaps needed at some point, but now there is no need in this.

See the next diff that refactors InferredNullability so we store all
joinee results there.

Reviewed By: artempyanykh

Differential Revision: D24621413

fbshipit-source-id: d6f406b87

infer/src/nullsafe/InferredNullability.ml

@@ -39,9 +39,9 @@ let join t1 t2 =
         t2.origin
     | false, false | true, true ->
         (* Nullability is not fully determined by neither t1 nor t2
-           Let TypeOrigin logic to decide what to prefer in this case.
+           Picking the left one for stability
         *)
-        TypeOrigin.join t1.origin t2.origin
+        t1.origin
   in
   {nullability= joined_nullability; origin= joined_origin}
 

infer/src/nullsafe/typeOrigin.ml

@@ -194,13 +194,3 @@ let get_description origin =
   (* A technical origin *)
   | OptimisticFallback ->
       None
-
-
-let join o1 o2 =
-  match (o1, o2) with
-  | Field _, (NullConst _ | NonnullConst _ | CurrMethodParameter _ | This | MethodCall _ | New) ->
-      (* low priority to Field, to support field initialization patterns *)
-      o2
-  | _ ->
-      (* left priority *)
-      o1

infer/src/nullsafe/typeOrigin.mli

@@ -52,8 +52,5 @@ val get_nullability : t -> Nullability.t
 val get_description : t -> string option
 (** Get a description to be used for error messages. *)
 
-val join : t -> t -> t
-(** Join with left priority *)
-
 val to_string : t -> string
 (** Raw string representation. *)