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

@@ -7,18 +7,17 @@
   - 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'}
-          = Invoice.human_attribute_name(:created_at)
-        .controls.control-text#article_fc_price
-          = format_date @invoice.created_at
-      .control-group
-        %label.control-label{for: 'created_by'}
-          = Invoice.human_attribute_name(:created_by)
-        .controls.control-text#article_fc_price
-          = show_user @invoice.created_by
+  .fold-line
+    .control-group
+      %label.control-label{for: 'created_at'}
+        = Invoice.human_attribute_name(:created_at)
+      .controls.control-text#article_fc_price
+        = format_date @invoice.created_at
+    .control-group
+      %label.control-label{for: 'created_by'}
+        = Invoice.human_attribute_name(:created_by)
+      .controls.control-text#article_fc_price
+        = show_user @invoice.created_by
   = f.association :supplier, collection: Supplier.order(:name), hint: false
   = f.input :number
   = f.input :date, as: :date_picker