Fix double <nav> tag in valuation menu

We were defining a <nav> tag which was already defined in the admin
layout.

app/views/valuation/_menu.html.erb

@@ -1,4 +1,3 @@
-<nav class="admin-sidebar">
 <ul id="valuation_menu">
   <li>
     <%= link_to t("valuation.menu.title"), valuation_root_path %>
@@ -10,4 +9,3 @@
     </li>
   <% end %>
 </ul>
-</nav>