Purge "event" from the UI (and code) unless we're talking about event.module

Another followup issue from #86462: add backend support for cck date fields and #328181: Allow both event and CCK date to work simultaneously...

I'm trying to purge "event" from the signup codebase and UI unless it's actually referring to event.module.

In user facing text, we should use node_get_types('name') anyway, so if you have signup-enabled event nodes called "Gig" or "Rehearsal" or whatever, we print the right thing.