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

Merges [42478] to the 4.0 branch.
Fixes #42720 for 4.0.

Built from https://develop.svn.wordpress.org/branches/4.0@42486


git-svn-id: http://core.svn.wordpress.org/branches/4.0@42315 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/update-core.php

@@ -659,6 +659,9 @@ $_old_files = array(
 'wp-includes/js/tinymce/plugins/paste/editor_plugin_src.js',
 'wp-includes/js/tinymce/plugins/paste/pastetext.htm',
 'wp-includes/js/tinymce/langs/wp-langs.php',
+// 4.9.2
+'wp-includes/js/mediaelement/flashmediaelement.swf',
+'wp-includes/js/mediaelement/silverlightmediaelement.xap',
 );
 
 /**