[optimization] update_post_caches() unnecessarily includes posts table in SQL

[coffee2code]

In wp-includes/functions.php, the function update_post_caches() appears to unnecessarily include the posts table in the SQL statement. The function already has a list of post IDs ($post_ids, assembled from passed in $posts) for which to find matching $wpdb->post2cat.post_id rows to update the posts category cache (what this SQL is there for).

Nice patch