Event model improvements

- Drop selected_event_schedule function
- Rename scheduled_room and scheduled_start_time to room and time as if there is no selected_schedule, the event is not scheduled.

app/views/conference_registrations/_registration_info.html.haml

@@ -11,7 +11,7 @@
         .text-muted
           = registered_text(event)
           - if event.scheduled?
-            (Scheduled on: #{event.scheduled_start_time.to_date})
+            (Scheduled on: #{event.time.to_date})
       %br