msglist [nfc]: Cut already-broken logic for slightly less padding at bottom

This `i == 1` condition was never true, because in that situation the
caller would build a MarkAsReadWidget instead of calling this method.

This 8px vs. 11px distinction dates back to the prototype: 731b199
made it 8px instead of 0px, and the distinction itself goes back to
the commit
  9916194 msglist: Start on rendering messages
in the prototype's first hours.

The logic that drove it, though, became fragile with e7fe06c which
changed it from "i == 0" (the end of the list, OK that's fairly
canonical as a special value) to "i == 1" (more arbitrary).  So then
it naturally got broken a little later, in 56ab395 in 2023-11, and
it's been broken ever since: we just always show 11px of padding here.

We might further change the layout in the future, but if we do we'll
fix it forward starting from the behavior the app has already had for
over a year.

Author: gnprice

lib/widgets/message_list.dart

@@ -622,7 +622,7 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
           if (i == 2) return TypingStatusWidget(narrow: widget.narrow);
 
           final data = model!.items[length - 1 - (i - 3)];
-          return _buildItem(zulipLocalizations, data, i);
+          return _buildItem(zulipLocalizations, data);
         }));
 
     if (!ComposeBox.hasComposeBox(widget.narrow)) {
@@ -659,7 +659,7 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
       ]);
   }
 
-  Widget _buildItem(ZulipLocalizations zulipLocalizations, MessageListItem data, int i) {
+  Widget _buildItem(ZulipLocalizations zulipLocalizations, MessageListItem data) {
     switch (data) {
       case MessageListHistoryStartItem():
         return Center(
@@ -685,7 +685,7 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
         return MessageItem(
           key: ValueKey(data.message.id),
           header: header,
-          trailingWhitespace: i == 1 ? 8 : 11,
+          trailingWhitespace: 11,
           item: data);
     }
   }