No subtitle instead of empty subtitle

Otherwise Pod::To::BigPage seems to be unhappy.

Author: AlexDaniel

manage-page-order.p6

@@ -401,7 +401,6 @@ sub write-group-page($group-char, $title, $page-order) {
     $fh.print: qq:to/HERE/;
         =begin pod :page-order<{$page-order}>
         =TITLE $title
-        =SUBTITLE
         =end pod
         HERE