Add this code in functions.php
add_action('admin_init', 'wpse_87339_admin_init');
function wpse_87339_admin_init() {
wp_enqueue_script('wpse_87339_admin', get_stylesheet_directory_uri() . '/js/admin.js', array('jquery'));
}
and create new js file name admin.js and then add the below code in it
jQuery(document).ready(function($) {
// Remove all .postbox classes from sidebar
$('#side-sortables .postbox').removeClass('postbox');
// Remove .postbox from specific metabox (page attributes)
$('#pageparentdiv').removeClass('postbox');
});