fix: remove duplicate apiKeys field from ReferralCustomer

ReferralCustomer extends BaseUser, which now has the apiKeys field.
Having apiKeys in both classes causes a Gson duplicate field error
during JSON deserialization.

Author: lanej

src/main/java/com/easypost/model/ReferralCustomer.java

@@ -1,10 +1,8 @@
 package com.easypost.model;
 
-import java.util.List;
-
 import lombok.Getter;
 
 @Getter
 public class ReferralCustomer extends BaseUser {
-    private List<ApiKey> apiKeys;
+    // apiKeys is inherited from BaseUser
 }