How to delete the website field in a WordPress post comment?

Simple! Just add this code in the functions.php file located in your active theme folder.

<?php
add_filter('comment_form_default_fields', 'unset_url_field');
function unset_url_field($fields){
    if(isset($fields['url']))
       unset($fields['url']);
       return $fields;
}
add_filter( 'excerpt_length', function( $length ) { return 20; } );