redmine/app/views/layouts/admin.html.erb

<% unless controller_name == 'admin' && action_name == 'index' %>
  <% content_for :sidebar do %>
    <h3><%=l(:label_administration)%></h3>
    <%= render :partial => 'admin/menu' %>
  <% end %>
<% end %>

<%= render template: 'layouts/base' %>