如果其他人偶然发现了这种问题，下面是我的解决方案：（无需管理任何代谢箱等）

add_action(\'categorycourses_add_form_fields\',\'categorycourses_edit_form_fields\');
add_action(\'categorycourses_edit_form_fields\',\'categorycourses_edit_form_fields\');

//Function only showing parent categories when editing or adding new category-courses
function categorycourses_edit_form_fields() {
    ?>
    <script type="text/javascript">
    jQuery(document).ready( function($) {
       var level = 1;
       //Go down in hierarchy from level1 and remove each level until there are no levels anymore
       //Don\'t do anything with level-0 because we want to show parent-categories
       while ( level < $("form .form-field #parent").find(".level-" +  level).length>0 ) { 
            $("form .form-field #parent").find(".level-1").remove(); //Remove level
       }
    });
    </script>    
<?php
}

add_action(\'{taxonomy}_add_form_fields\',\'{taxonomy}_edit_form_fields\');
add_action(\'{taxonomy}_edit_form_fields\',\'{taxonomy}_edit_form_fields\');