How to eliminate the Web Address / Web Link textbox from comment forms

You made a mistake when put your functions.php file into mu-plugins directory. Why? Because your file rendered above is not a plugin, it is just a script. WordPress looks for a plugin inside mu-plugins directory. So you have to do one of two things:

• Add plugin header into functions.php file
• Move functions.php file into your theme root folder (+ merge it with existing one)

Plugin header:

<?php
/**
 * Plugin Name: Name Of The Plugin
 * Plugin URI: http://URI_Of_Page_Describing_Plugin_and_Updates
 * Description: A brief description of the Plugin.
 * Version: The Plugin's Version Number, e.g.: 1.0
 * Author: Name Of The Plugin Author
 * Author URI: http://URI_Of_The_Plugin_Author
 * License: A "Slug" license name e.g. GPL2
 */

// to remove website textbox from Comments form
add_filter('comment_form_default_fields', 'url_filtered');
function url_filtered($fields) {
  if(isset($fields['url']))
  unset($fields['url']);
  return $fields;
}