api: Remove sender_id fallback for UnreadMessagesSnapshot.otherUserId, relying on server 5+, FL 119+

See "Feature level 119" from Zulip API changelog:

   https://zulip.com/api/changelog

See also:

  https://chat.zulip.org/#narrow/stream/412-api-documentation/topic/register.3A.20When.20was.20.60unread_msgs.2Epms.5B.5D.2Eother_user_id.60.20added.3F

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

Author: PIG208

lib/api/model/initial_snapshot.dart

@@ -270,15 +270,9 @@ class UnreadMessagesSnapshot {
 /// An item in [UnreadMessagesSnapshot.dms].
 @JsonSerializable(fieldRename: FieldRename.snake)
 class UnreadDmSnapshot {
-  @JsonKey(readValue: _readOtherUserId)
   final int otherUserId;
   final List<int> unreadMessageIds;
 
-  // TODO(server-5): Simplify away.
-  static dynamic _readOtherUserId(Map<dynamic, dynamic> json, String key) {
-    return json[key] ?? json['sender_id'];
-  }
-
   UnreadDmSnapshot({
     required this.otherUserId,
     required this.unreadMessageIds,