Fix post_class filter expecting too many args

Current Wordpress offers only one argument to post_class filters. This patch avoids a warning in the error log when the filter expects three but doesn't get them.

The second one wasn't being used so removing it does no harm. The third ($post_id) was being used but Wordpress documentation for post_class filters at http://codex.wordpress.org/Function_Reference/post_class#Add_Classes_By_Filters shows it is available as $post->ID.