bbp_notify_forum_subscribers: $user_id leftover

[wpdennis]

While applying filters in ​bbp_notify_forum_subscribers() $user_id isnt' available in the context and for the filter anymore:

$message = apply_filters( 'bbp_forum_subscription_mail_message', $message, $topic_id, $forum_id, $user_id );
	        if ( empty( $message ) ) {
	                return;
	        }

and

 $subject = apply_filters( 'bbp_forum_subscription_mail_title', '[' . $blog_name . '] ' . $topic_title, $topic_id, $forum_id, $user_id );
	        if ( empty( $subject ) ) {
	                return;
	        }

[netweb]

Duplicate: #2620