Cleanup ding_event template so ding_place2book module can show its field

The change in https://github.com/ding2tal/ddbasic/commit/1fc3218ddf8337dfd5e71b4b5ea6d3cffe564213 did not remove all references to field_place2book_tickets. As a consequence, that field will not show up in any way. This PR cleans up the template to finish up.