api: Drop legacy support for prev_subject and topic, relying on server 5+, FL 118+

"topic" is no longer optional when the topic was edited, we also don't
need to expect "prev_subject" as opposed to "prev_topic".

See "Feature level 118" from Zulip API changelog:

   https://zulip.com/api/changelog

See also:

   https://zulip.com/api/get-messages#response

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/api/model/model.dart

@@ -824,16 +824,12 @@ enum MessageEditState {
         continue;
       }
 
-      // TODO(server-5) prev_subject was the old name of prev_topic on pre-5.0 servers
-      final prevTopic = (entry['prev_topic'] ?? entry['prev_subject']) as String?;
+      final prevTopic = entry['prev_topic'] as String?;
       final topic = entry['topic'] as String?;
-      if (prevTopic != null) {
-        // TODO(server-5) pre-5.0 servers do not have the 'topic' field
-        if (topic == null) {
-          hasMoved = true;
-        } else {
-          hasMoved |= !topicMoveWasResolveOrUnresolve(topic, prevTopic);
-        }
+      if (topic != null || prevTopic != null) {
+        // Both are present if the topic was edited
+        assert(topic != null && prevTopic != null);
+        hasMoved |= !topicMoveWasResolveOrUnresolve(topic!, prevTopic!);
       }
     }
 

test/api/model/model_test.dart

@@ -279,16 +279,6 @@ void main() {
         checkEditState(MessageEditState.edited,
           [{'prev_content': 'old_content'}]);
       });
-
-      test("'prev_topic' present without the 'topic' field -> moved", () {
-        checkEditState(MessageEditState.moved,
-          [{'prev_topic': 'old_topic'}]);
-      });
-
-      test("'prev_subject' present from a pre-5.0 server -> moved", () {
-        checkEditState(MessageEditState.moved,
-          [{'prev_subject': 'old_topic'}]);
-      });
     });
 
     group('topic resolved in edit history', () {