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.

tag/edit.php

@@ -69,7 +69,6 @@ $PAGE->set_subpage($tag->id);
 $PAGE->set_context($systemcontext);
 $PAGE->set_blocks_editing_capability('moodle/tag:editblocks');
 $PAGE->set_pagelayout('standard');
-$PAGE->set_secondary_navigation(false);
 
 $tagname = $tag->get_display_name();
 $tagcollid = $tag->tagcollid;