You can use the nav_menu_css_class
filter to remove the classes that you want from your menu-item.
See the example below, to remove ALL the CSS classes that are appended to you menu items:
add_filter('nav_menu_css_class','remove_nav_menu_classes');
function remove_nav_menu_classes($classes) {
return array();
}
Do a print_r($classes)
if you want to know which classes are active in your menu.