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

@@ -1,5 +1,4 @@
-<nav class="admin-sidebar">
-  <ul id="valuation_menu">
+<ul id="valuation_menu">
   <li>
     <%= link_to t("valuation.menu.title"), valuation_root_path %>
   </li>
@@ -9,5 +8,4 @@
       <%= link_to t("valuation.menu.budgets"), valuation_budgets_path, class: "budgets-link" %>
     </li>
   <% end %>
-  </ul>
-</nav>
+</ul>