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.


git-svn-id: https://develop.svn.wordpress.org/trunk@52267 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/feed/atom.php

@@ -99,7 +99,6 @@ class Tests_Feed_Atom extends WP_UnitTestCase {
 		// Verify attributes.
 		$this->assertSame( 'http://www.w3.org/2005/Atom', $atom[0]['attributes']['xmlns'] );
 		$this->assertSame( 'http://purl.org/syndication/thread/1.0', $atom[0]['attributes']['xmlns:thr'] );
-		$this->assertSame( site_url( '/wp-atom.php' ), $atom[0]['attributes']['xml:base'] );
 
 		// Verify the <feed> element is present and contains a <title> child element.
 		$title = xml_find( $xml, 'feed', 'title' );