Remove useless if in invoices form

The created_at field has a value all the time, so the check if it is set
make no sense since it evaluates to true all the time.

app/views/finance/invoices/_form.html.haml

@@ -7,7 +7,6 @@
   - if @invoice.orders.first
     %p= t('finance.invoices.linked', what_link: link_to(t('finance.invoices.linked_order'), new_finance_order_path(order_id: @invoice.orders.first.id))).html_safe
 
-  - if @invoice.created_at
   .fold-line
     .control-group
       %label.control-label{for: 'created_at'}