Revert back to using deltas but don't do any fancy calulations

  Go back to using the start/end deltas but just pick one or the other
  depending on which one has changed. Don't do fancy calulations to get
  the duration because it seems the durationDelta isn't really a duration
  delta at all...

  Instead, it's simply the delta of the time that has changed. The
  variable name should reflect this but I think I'm done for now if that
  makes sense :).

src/View.ts

@@ -660,10 +660,7 @@ export default abstract class View extends InteractiveDateComponent {
       this.calendar
     )
 
-    let durationDelta = this.diffDates(
-      eventInstance.dateProfile.end,
-      eventInstance.dateProfile.start
-    )
+    let durationDelta = eventMutation.dateMutation.endDelta || eventMutation.dateMutation.startDelta
 
     this.triggerEventResize(
       eventInstance,