wp_list_pages Styling help

I have created my own theme. Looking at the source code I couldn’t help but notices that, for example, class=”page_item page-item-12 current_page_item” is being added to the <?php wp_list_pages(‘title_li=’); ?> function I am using to output the pages. This styling in unneeded so how do I remove it?

Here is an example of my source code:

<ul id="mainMenu">
     <li><a href="index.php">Homepage</a></li>
     <li class="page_item page-item-2"><a href="http://localhost/wordpress/?page_id=2">Sample Page</a></li>
     <li class="page_item page-item-12 current_page_item"><a href="http://localhost/wordpress/?page_id=12">Test Page</a></li>
</ul>