remove firefox download button for topic nav

kitsune/products/jinja2/products/includes/topic_documents.html

@@ -2,7 +2,6 @@
   <div class="sumo-article-header--text">
 
     <div class="documents-product-title">
-      {{ download_firefox() }}
       <h1 class="topic-title is-subtopic sumo-page-heading">
         {{ pgettext('DB: products.Topic.title', topic.title) }}
       </h1>
@@ -15,4 +14,4 @@
       </p>
     {% endif %}
   </div>
-</header>
+</header>