api: Tighten value type on ChannelUpdateEvent for canRemoveSubscribersGroup

This property on streams/channels is documented as type "integer":
  https://zulip.com/api/get-streams#response

(And the same goes for its former name of canRemoveSubscribersGroupId.)

So when the server supports this property at all, and therefore might
send us an event for it, the value it supplies will be non-null.
The corresponding field on ZulipStream is nullable only because of
servers that don't yet support the property.

Author: gnprice

lib/api/model/events.dart

@@ -433,6 +433,7 @@ class ChannelUpdateEvent extends ChannelEvent {
         return ChannelPostPolicy.fromApiValue(value as int);
       case ChannelPropertyName.canRemoveSubscribersGroup:
       case ChannelPropertyName.canRemoveSubscribersGroupId:
+        return value as int;
       case ChannelPropertyName.streamWeeklyTraffic:
         return value as int?;
       case null: