不要な変数定義を削除する

https://github.com/LeeDDHH/qiita_daily/blob/57717dc7e6bbc1851ea8a249f60947e2fe44642d/get-article.php#L30-L31

## 🙅 

```php
$data = json_decode($json);
return $data;
```

```php
$articleFile = file_get_contents($url,false, $context);
return $articleFile;
```

## 🙆 

```php
return json_decode($json);
```