MDL-74386 navigation: Remove redundant set_secondary_navigation() calls

Removes $PAGE->set_secondary_navigation(false) calls which certain pages
were using to prevent displaying secondary navigation menu in the system
context. This logic is now generally handled within the secondary
navigation class.

error/index.php

@@ -36,7 +36,6 @@ $PAGE->set_context($context);
 $PAGE->set_title($title);
 $PAGE->set_heading($title);
 $PAGE->navbar->add($title);
-$PAGE->set_secondary_navigation(false);
 
 // This allows the webserver to dictate wether the http status should remain
 // what it would have been, or force it to be a 404. Under other conditions