fix(core): remove unnecessary role=banner on top level header

By default, the HTML's `<header>` element has an identical meaning to the banner landmark,
unless it is a descendant of `<aside>`, `<article>`, `<main>`, `<nav>`, or `<section>`.

Signed-off-by: Grigorii K. Shartsev <me@shgk.me>

core/templates/layout.guest.php

@@ -33,7 +33,7 @@ p($theme->getTitle());
 		<div class="wrapper">
 			<div class="v-align">
 				<?php if ($_['bodyid'] === 'body-login'): ?>
-					<header role="banner">
+					<header>
 						<div id="header">
 							<div class="logo"></div>
 						</div>

core/templates/layout.user.php

@@ -56,7 +56,7 @@ p($theme->getTitle());
 			<?php if ($_['id-app-navigation'] !== null) { ?><a href="<?php p($_['id-app-navigation']); ?>" class="button primary skip-navigation"><?php p($l->t('Skip to navigation of app')); ?></a><?php } ?>
 		</div>
 
-		<header role="banner" id="header">
+		<header id="header">
 			<div class="header-left">
 				<a href="<?php print_unescaped($_['logoUrl'] ?: link_to('', 'index.php')); ?>"
 					aria-label="<?php p($l->t('Go to %s', [$_['logoUrl'] ?: $_['defaultAppName']])); ?>"