Fix validation failure in assistant-csv-trans-import.glade

- Redo change from 0d8640f
 - (Glade bug?)

$ gtk-builder-tool validate gnucash/gtkbuilder/assistant-csv-trans-import.glade
Gtk-CRITICAL **: 21:57:40.154: gtk_assistant_set_page_has_padding: assertion 'child != NULL' failed

gnucash/gtkbuilder/assistant-csv-trans-import.glade

@@ -1145,9 +1145,6 @@ More information can be displayed by using the help button.</property>
       <object class="GtkBox">
         <property name="can-focus">False</property>
       </object>
-      <packing>
-        <property name="has-padding">False</property>
-      </packing>
     </child>
   </object>
 </interface>