Unable to modify comment_notes_before field in comment form

I am trying to remove

Your email address will not be published.

line from comment form in twentythirteen and twentyfourteen.

I have tried this:

function alter_comment_form_fields($fields){
    unset($fields['comment_notes_before']);
    return $fields;
}
add_filter('comment_form_default_fields','alter_comment_form_fields');

and this:

function alter_comment_form_fields($fields){
    $fields['comment_notes_before'] = '<p>Email address not required.</p>';
    return $fields;
}
add_filter('comment_form_default_fields','alter_comment_form_fields');

Nothing seems to work. I have also found this ticket
https://core.trac.wordpress.org/ticket/14510

Would appreciate if someone can point me to some direction.