functions.php

<?php

/**
 * Remove URL field from comment form 
 *
 */
function be_remove_url_from_comment_form( $fields ) {
  unset($fields['url']);
  return $fields;
}
add_filter( 'comment_form_default_fields', 'be_remove_url_from_comment_form' );

/**
 * Remove URL from existing comments 
 *
 */
function be_remove_url_from_existing_comments( $author_link, $author ) {
  return $author;
}
add_filter( 'get_comment_author_link', 'be_remove_url_from_existing_comments', 10, 2 );