Customize admin interface

I’m trying to remove some columns in admin, too many column make the layout look ugly and broke. I’m using this code below but it’s doesn’t work. Please help!

function wptutsplus_remove_testimonials_listing_tags( $columns ) {
    unset( $columns[ 'tags' ] );
    unset( $columns[ 'comments' ] );
    unset( $columns['author'] );
    return $columns;
}
add_action( 'manage_testimonials-widget_posts_columns', 'wptutsplus_remove_testimonials_listing_tags' );

http://wordpress.org/plugins/testimonials-by-woothemes/