unreads [nfc]: Remove some TODO(server-6) comments about fixed server bugs

Fixes-partly: zulip#1837

Author: chrisbobbe

lib/model/unreads.dart

@@ -533,11 +533,7 @@ class Unreads extends PerAccountStoreBase with ChangeNotifier {
     final topics = streams[streamId] ??= makeTopicKeyedMap();
     topics.update(topic,
       ifAbsent: () => messageIds,
-      // setUnion dedupes existing and incoming unread IDs,
-      // so we tolerate zulip/zulip#22164, fixed in 6.0
-      // TODO(server-6) remove 6.0 comment
-      (existing) => setUnion(existing, messageIds),
-    );
+      (existing) => setUnion(existing, messageIds));
   }
 
   /// Remove [idsToRemove] from [streams] and [dms].
@@ -636,10 +632,6 @@ class Unreads extends PerAccountStoreBase with ChangeNotifier {
   void _addAllInDm(QueueList<int> messageIds, DmNarrow dmNarrow) {
     dms.update(dmNarrow,
       ifAbsent: () => messageIds,
-      // setUnion dedupes existing and incoming unread IDs,
-      // so we tolerate zulip/zulip#22164, fixed in 6.0
-      // TODO(server-6) remove 6.0 comment
-      (existing) => setUnion(existing, messageIds),
-    );
+      (existing) => setUnion(existing, messageIds));
   }
 }

test/model/unreads_test.dart

@@ -1250,22 +1250,6 @@ void main() {
         checkMatchesMessages([message1, message2]);
       });
 
-      // TODO(server-6) remove mention of zulip/zulip#22164, fixed in 6.0
-      test('tolerates event pointing to DM/stream messages that are already unread (zulip/zulip#22164)', () {
-        final message1 = eg.streamMessage(id: 1, flags: []);
-        final message2 = eg.dmMessage(id: 2, from: eg.otherUser, to: [eg.selfUser], flags: []);
-
-        prepare();
-        fillWithMessages([message1, message2]);
-
-        model.handleUpdateMessageFlagsEvent(mkEvent([message1, message2]));
-        checkNotifiedOnce();
-
-        message1.flags.remove(MessageFlag.read);
-        message2.flags.remove(MessageFlag.read);
-        checkMatchesMessages([message1, message2]);
-      });
-
       test('tolerates "message details" missing', () {
         final stream = eg.stream();
         const topic = 'a';