remove deprecated call to update button

sessions.php

@@ -59,7 +59,6 @@ $PAGE->set_url($att->url_sessions(array('action' => $pageparams->action)));
 $PAGE->set_title($course->shortname. ": ".$att->name);
 $PAGE->set_heading($course->fullname);
 $PAGE->set_cacheable(true);
-$PAGE->set_button($OUTPUT->update_module_button($cm->id, 'attendance'));
 $PAGE->navbar->add($att->name);
 
 $currenttab = attendance_tabs::TAB_ADD;