Replace service desk promotion clear button FA icon with GlIcon close

app/views/shared/promotions/_promote_servicedesk.html.haml

 .user-callout.promotion-callout.js-service-desk-callout#promote_service_desk{ data: { uid: 'promote_service_desk_dismissed' } }
   .bordered-box.content-block
     %button.btn.btn-default.close.js-close-callout{ type: 'button', 'aria-label' => 'Dismiss Service Desk promotion' }
-      = icon('times', class: 'dismiss-icon', 'aria-hidden' => 'true')
+      = sprite_icon('close', size: 16, css_class: 'dismiss-icon')
     .svg-container
       = custom_icon('icon_service_desk')
     .user-callout-copy