Get rid of JsonApiResponse class

With the JSON-API library being updated, we can just make use of
Diactoros' JSON response class.

src/Api/JsonApiResponse.php

@@ -1,14 +0,0 @@
-<?php namespace Flarum\Api;
-
-use Tobscure\JsonApi\Document;
-use Zend\Diactoros\Response;
-
-class JsonApiResponse extends Response
-{
-    public function __construct(Document $document)
-    {
-        parent::__construct('php://memory', 200, ['content-type' => 'application/vnd.api+json']);
-
-        $this->getBody()->write($document);
-    }
-}