The sitemap added by most of the plugins (such as Google sitemaps or YOAST SEO pack) is a virtual file added to your websites by the plugin. This file doesn’t physically exist, therefore modifying it is not an option for you.
There might be 2 things that you can do about it,
Either find the php file that is generating the XML and modify it, or maybe add a filter if it’s supported. In your case, the file is located in /plugins/all-in-one-seo-pack/inc/sitemap-xsl.php.
OR
Find the option to demote your URL from the XML list. This option may not exist on every plugin.