[templates][actstream] Remove URL_PREFIX

It's redundant as the URLs generated shall handle that

beat/web/templates/actstream/action.html

@@ -32,9 +32,9 @@
         </td>
         <td style="border-top:0;padding:0;padding-left:0.5em">
           {% if action.actor.get_absolute_url %}
-          <a href="{{ URL_PREFIX }}{{ action.actor.get_absolute_url }}">
+          <a href="{{ action.actor.get_absolute_url }}">
           {% else %}
-          <a href="{{ URL_PREFIX }}{{ action.actor_url }}">
+          <a href="{{ action.actor_url }}">
           {% endif %}
             {{ action.actor.first_name }} ({{ action.actor }})
           </a>