How do I remove a menu item on a module?
This is an option in a form. You can use hook form alter in your custom module to hide it. For example

function my_module_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'SOME_FORM_ID') { 
    // you need to find out the form id . You can do so by adding a 
    // print line or (dpm($form_id)) in the ..._form function in the Course module.
    $form['course']['duration']['#access'] = FALSE;
   }
 }