Purge Transients by @rarst

add_action( 'wp_scheduled_delete', 'delete_expired_db_transients' );
function delete_expired_db_transients() {

    if ( wp_using_ext_object_cache() ) { // magic
        return;
    }

    global $wpdb;

    $time = isset( $_SERVER['REQUEST_TIME'] ) ? (int) $_SERVER['REQUEST_TIME'] : time();
    $expired = $wpdb->get_col( "SELECT option_name FROM {$wpdb->options} WHERE option_name LIKE '_transient_timeout%' AND option_value < {$time};" );

    foreach ( $expired as $transient ) {
        $key = str_replace( '_transient_timeout_', '', $transient );
        delete_transient( $key );
    }

}
// by @rarst