MDL-68135 output: remove the presentation role from user pic

"presentation" is not a valid role for the img element.
Moreover, it doesn't make sense to have both role="presentation" and
aria-hidden="true" for an element at the same time.
For more info please see:
https://timwright.org/blog/2016/11/19/difference-rolepresentation-aria-hiddentrue/

lib/outputrenderers.php

@@ -2526,7 +2526,6 @@ class core_renderer extends renderer_base {
 
         $attributes = array('src' => $src, 'class' => $class, 'width' => $size, 'height' => $size);
         if (!$userpicture->visibletoscreenreaders) {
-            $attributes['role'] = 'presentation';
             $alt = '';
             $attributes['aria-hidden'] = 'true';
         }