MDL-70829 mod_workshop: Mark up decorative images properly

* Decorative images should have an empty alt text and there's no need
to set a presentation role.

mod/workshop/renderer.php

@@ -308,7 +308,7 @@ protected function render_workshop_user_plan(workshop_user_plan $plan) {
 
             if ($phase->active) {
                 // Mark the section as the current one.
-                $icon = $this->output->pix_icon('i/marked', '', 'moodle', ['role' => 'presentation']);
+                $icon = $this->output->pix_icon('i/marked', '');
                 $actions .= get_string('userplancurrentphase', 'workshop').' '.$icon;
 
             } else {