Feeds: Remove reference to `wp-atom.php` in Atom feeds `xml:base` attribute.

This change removes the optional `xml:base` attribute for Atom based feeds as `wp-atom.php` file was removed in [18540] (WordPress 3.3).

Follow-up to [18540].

Props tw2113, audrasjb, mukesh27.
Fixes #47955.

Built from https://develop.svn.wordpress.org/trunk@52267


git-svn-id: http://core.svn.wordpress.org/trunk@51859 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/feed-atom.php

@@ -17,7 +17,6 @@ do_action( 'rss_tag_pre', 'atom' );
 	xmlns="http://www.w3.org/2005/Atom"
 	xmlns:thr="http://purl.org/syndication/thread/1.0"
 	xml:lang="<?php bloginfo_rss( 'language' ); ?>"
-	xml:base="<?php bloginfo_rss( 'url' ); ?>/wp-atom.php"
 	<?php
 	/**
 	 * Fires at end of the Atom feed root to add namespaces.

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-52266';
+$wp_version = '5.9-alpha-52267';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.