Recent Posts widget should set 'no_found_rows' => true
|Reported by:||scribu||Owned by:||ryan|
WP_Widget_Recent_Posts calls WP_Query, but doesn't need the total number of posts.
Therefore, it should set 'no_found_rows' => true to make the query faster.
Even with #17195, this should still be done, to avoid SQL_FOUND_ROWS.