Fix crash when scrolling through Local/Federated timeline with VoiceOver

It seems that the accessibility scroll mechanism does something like:
1. Find the next IndexPath to focus
2. Scroll to make it visible
3. Focus that cell

But because the timeline description cell is removed during the scroll,
the IndexPath that the accessibility system wants to focus becomes
invalid between steps 2 and 3, causing a crash when trying to focus it.

As a workaround, only remove the timeline description _item_ rather than
the header section so that section indices aren't affected.

Closes #290

Tusker/Screens/Timeline/TimelineViewController.swift

@@ -345,7 +345,7 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
     
     private func removeTimelineDescriptionCell() {
         var snapshot = dataSource.snapshot()
-        snapshot.deleteSections([.header])
+        snapshot.deleteItems([.publicTimelineDescription])
         dataSource.apply(snapshot, animatingDifferences: true)
         isShowingTimelineDescription = false
     }