On my site I have disabled most all features for Events in regards to apps.
This includes both Discussions and News/Announcements. Unfortunately when I go to edit the custom fields in the event categories and I remove the discussions and news "mandatory fields" it will not allow me to save the category.
If the apps aren't enabled the fields should in my opinion no longer be visible. However, it would be just as well to make them non-core or non-mandatory.
Thanks,
James