Make order customer email links consistent

Previously the order table would show user account links for
users who had accounts, and mailto links for those who did not.
This inconsistent link behavior is replaced with plaintext for
users who do not have accounts.

backend/app/views/spree/admin/orders/index.html.erb

@@ -184,7 +184,7 @@
           <% if order.user %>
             <%= link_to order.email, edit_admin_user_path(order.user) %>
           <% else %>
-            <%= mail_to order.email %>
+            <%= order.email %>
           <% end %>
         </td>
         <td class="align-right"><%= order.display_total.to_html %></td>