artists/summary: remove list of top source domains.

Remove the list of most-used source domains from artist summaries. This
took up a lot of space and usually wasn't very useful. It was also slow.
We had to calculate this on every artist tag search so we could display
it in the Artist tab, even though usually the user didn't open the tab.

app/views/artists/_summary.html.erb

@@ -24,14 +24,6 @@
       <% end %>
     </li>
   <% end %>
-  <% if artist.domains.any? %>
-    <li><strong>Domains</strong></li>
-    <ul class="list-bulleted">
-      <% artist.domains.each do |url, count| %>
-        <li><%= url %>: <%= count %></li>
-      <% end %>
-    </ul>
-  <% end %>
   <% if artist.urls.present? %>
     <li><strong>URLs</strong></li>
     <ul>