Remove support for XHTML markup

[salcode]

Bootstrap Genesis is an HTML5 theme. It does not make sense to define XHTML values.

e.g. in lib/pagination-prev-next.php we have the following code

    $nav = genesis_markup( array(
        'html5'   => '<nav %s><ul class="pager">',
        'xhtml'   => '<div class="navigation"><ul class="pager">',
        'context' => 'archive-pagination',
        'echo'    => false,
    ) );

and we can remove the xhtml line