External Libraries: Remove unnecessary / obsoleted MediaElement.js files.

Merges [42478] to the 4.6 branch.
Fixes #42720 for 4.6.

Built from https://develop.svn.wordpress.org/branches/4.6@42480


git-svn-id: http://core.svn.wordpress.org/branches/4.6@42309 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/update-core.php

@@ -705,6 +705,9 @@ $_old_files = array(
 'wp-includes/theme-compat/comments-popup.php',
 // 4.6
 'wp-admin/includes/class-wp-automatic-upgrader.php', // Wrong file name, see #37628.
+// 4.9.2
+'wp-includes/js/mediaelement/flashmediaelement.swf',
+'wp-includes/js/mediaelement/silverlightmediaelement.xap',
 );
 
 /**