Remove NewsletterPost model

[MAILPOET-3027]

lib/Models/NewsletterPost.php

@@ -1,13 +0,0 @@
-<?php
-
-namespace MailPoet\Models;
-
-/**
- * @property int $newsletterId
- * @property int $postId
- * @property string $updatedAt
- */
-class NewsletterPost extends Model {
-  public static $_table = MP_NEWSLETTER_POSTS_TABLE; // phpcs:ignore PSR2.Classes.PropertyDeclaration
-
-}