fix invalid power level creation

oops, misread spec and now this breaks rooms on Synapse

lib/models/content/power_levels.dart

@@ -26,10 +26,7 @@ sealed class PowerLevelsContent extends Content with _$PowerLevelsContent {
 
 @Freezed(toJson: false, fromJson: false)
 @JsonSerializable()
-class const Notifications({
-  final int room = 50,
-  final IMap<String, int> other = const .empty(),
-}) with _$Notifications {
+class const Notifications({final int room = 50}) with _$Notifications {
   Map<String, Object?> toJson() => _$NotificationsToJson(this);
 
   factory Notifications.fromJson(Map<String, Object?> json) =>