I have never used this feature (vote up if you did not either ;-) ).
You can remove it with an advanced theme or layer:
function head_script(){
qa_html_theme_base::head_script();
// question edit state, remove 'edit silent' checkbox
if($this->template=='question' && strpos(qa_get_state(),'edit')!==false) {
unset($this->content['form_q_edit']['fields']['silent']);
}
}
By the way, maybe there is an easier way to hide this field. If you know it, let me/us know.
PS: It has no CSS id, so CSS cannot be used...