twig: remove aria-labelledby attribute and h2 it is referencing as it…

… causes accessbility issues when an h1 is present after the h2, replace at with aria-label instead. For #539.

src/templates/partials/school-header.twig

@@ -30,8 +30,7 @@
 
           <div class="school-nav__menu">
 
-            <nav aria-labelledby="midd-main-nav-label">
-              <h2 class="sr-only" id="midd-main-nav-label">Main navigation</h2>
+            <nav aria-label="Main navigation">
               <ul class="school-nav__list">
                 {% for item in site_nav.items %}
                   <li class="school-nav__item">