Improve performance in _fullcalendar_date_all_day_field()

_fullcalendar_date_all_day_field() is a wrapper around the deprecated date_field_all_day() and the current date_all_day_field(). It prefers the old function. However, it would be better to try the newer function first as it contains a faster code path to determine whether a field supports all-day events. In my case, I was using fullcalendar on a field that doesn't support all-day events anyway, and date_all_day_field() is much faster here.