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

Merges [42478] to the 4.2 branch.
Fixes #42720 for 4.2.

Built from https://develop.svn.wordpress.org/branches/4.2@42484


git-svn-id: http://core.svn.wordpress.org/branches/4.2@42313 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/update-core.php

@@ -694,7 +694,10 @@ $_old_files = array(
 'wp-includes/js/jquery/ui/jquery.ui.tabs.min.js',
 'wp-includes/js/jquery/ui/jquery.ui.tooltip.min.js',
 'wp-includes/js/jquery/ui/jquery.ui.widget.min.js',
-'wp-includes/js/tinymce/skins/wordpress/images/dashicon-no-alt.png'
+'wp-includes/js/tinymce/skins/wordpress/images/dashicon-no-alt.png',
+// 4.9.2
+'wp-includes/js/mediaelement/flashmediaelement.swf',
+'wp-includes/js/mediaelement/silverlightmediaelement.xap',
 );
 
 /**