[ECP-9852] Remove the helper function declaration from the interface (#3129)

Author: candemiralp

Api/Data/NotificationInterface.php

@@ -234,9 +234,4 @@ public function getUpdatedAt(): ?string;
      * @return NotificationInterface
      */
     public function setUpdatedAt(string $timestamp): NotificationInterface;
-
-    /**
-     * @return string|null
-     */
-    public function getFormattedAmountCurrency(): ?string;
 }