This is the area where you will change the setRequred to false. I didn't include the whole php file here.    


        $element = new TextField('text');
        $element->setRequired(true); <==== set this line to false
        $element->setLabel($language->text('back2top', 'admin_display_text')); 
        $element->setDescription($language->text('back2top', 'admin_display_text_desc')); 
        $element->setValue($config->getValue('back2top', 'text'));
        $adminForm->addElement($element);