Remove use of global $id from comment_form() and the media list table. see #24330.

git-svn-id: https://develop.svn.wordpress.org/trunk@24337 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/comment-template.php

@@ -1597,10 +1597,8 @@ function wp_list_comments($args = array(), $comments = null ) {
  * @return void
  */
 function comment_form( $args = array(), $post_id = null ) {
-	global $id;
-
 	if ( null === $post_id )
-		$post_id = $id;
+		$post_id = get_the_ID();
 	else
 		$id = $post_id;