MDL-73386 mod_quiz: Use plain text for quiz navigation fake block title

* No need to enclose the title of the quiz navigation fake block in
a span tag. The block title also gets appended to the skip link which
means that a block title with a span tag assigned with an identifier
can cause duplicate ID errors on the DOM.

mod/quiz/attemptlib.php

@@ -1889,8 +1889,7 @@ class quiz_attempt {
         $bc = new block_contents();
         $bc->attributes['id'] = 'mod_quiz_navblock';
         $bc->attributes['role'] = 'navigation';
-        $bc->attributes['aria-labelledby'] = 'mod_quiz_navblock_title';
-        $bc->title = html_writer::span(get_string('quiznavigation', 'quiz'), '', array('id' => 'mod_quiz_navblock_title'));
+        $bc->title = get_string('quiznavigation', 'quiz');
         $bc->content = $output->navigation_panel($panel);
         return $bc;
     }