remove deprecated call to update button

preferences.php

@@ -53,7 +53,6 @@ $PAGE->set_url($att->url_preferences());
 $PAGE->set_title($course->shortname. ": ".$att->name.' - '.get_string('settings', 'attendance'));
 $PAGE->set_heading($course->fullname);
 $PAGE->set_cacheable(true);
-$PAGE->set_button($OUTPUT->update_module_button($cm->id, 'attendance'));
 $PAGE->navbar->add(get_string('settings', 'attendance'));
 
 $errors = array();