The code is in wp-admin/includes/template.php, function screen_meta(). However, there’s no hook to disable the help button, so if you want to get rid of the button, you’d have to use CSS or JS.
If you choose CSS, a
body.index-php .screen-meta #contextual-help-link-wrap { display: none; }
should do.
If you just want to get rid of the content, use the 'contextual_help' filter and return an empty string:
add_filter('contextual_help', 'my_dashboard_help', 10, 2);
function my_dashboard_help($help, $screen_id) {
if ($screen_id == 'dashboard') $help = '';
return $help;
}