api: Drop is_cross_realm_bot fallback for User.isSystemBot, relying on server 5+, FL 83+

See "Feature level 83" from Zulip API changelog:

   https://zulip.com/api/changelog

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

Author: PIG208

lib/api/model/model.dart

@@ -221,7 +221,9 @@ class User {
   @JsonKey(readValue: _readProfileData)
   Map<int, ProfileFieldUserData>? profileData;
 
-  @JsonKey(readValue: _readIsSystemBot)
+  // This field is absent in `realm_users` and `realm_non_active_users`,
+  // which contain no system bots; it's present in `cross_realm_bots`.
+  @JsonKey(defaultValue: false)
   final bool isSystemBot;
 
   static Map<String, dynamic>? _readProfileData(Map<dynamic, dynamic> json, String key) {
@@ -233,14 +235,6 @@ class User {
     return (value != null && value.isNotEmpty) ? value : null;
   }
 
-  static bool _readIsSystemBot(Map<dynamic, dynamic> json, String key) {
-    // This field is absent in `realm_users` and `realm_non_active_users`,
-    // which contain no system bots; it's present in `cross_realm_bots`.
-    return (json[key] as bool?)
-        ?? (json['is_cross_realm_bot'] as bool?) // TODO(server-5): renamed to `is_system_bot`
-        ?? false;
-  }
-
   User({
     required this.userId,
     required this.deliveryEmail,

lib/api/model/model.g.dart

@@ -62,7 +62,6 @@ void main() {
 
     test('is_system_bot', () {
       check(mkUser({}).isSystemBot).isFalse();
-      check(mkUser({'is_cross_realm_bot': true}).isSystemBot).isTrue();
       check(mkUser({'is_system_bot': true}).isSystemBot).isTrue();
     });
   });