Stop duplicate entries for the invoice taxtable cell

When the ledger is loaded, the list for the taxtable combo is created
but as the item_list gui is not initialised yet a temporary list store
is used. This is done twice and hence end up with duplicate entries. To
fix this add the missing gtk_list_store_clear to
gnc_combo_cell_clear_menu to clear the temporary list store when the
item_list is not initialised.

gnucash/register/register-gnome/combocell-gnome.c

@@ -381,6 +381,8 @@ gnc_combo_cell_clear_menu (ComboCell * cell)
 
         unblock_list_signals (cell);
     }
+    else
+        gtk_list_store_clear (box->tmp_store);
 }
 
 void