deletePusher method
Variant of postPusher operation that deletes pushers by setting kind: null.
https://matrix.org/docs/spec/client_server/r0.6.1#post-matrix-client-r0-pushers-set
Implementation
Future<void> deletePusher(PusherId pusher) async {
  final data = PusherData.fromJson(pusher.toJson()).toJson();
  data['kind'] = null;
  await request(
    RequestType.POST,
    '/client/v3/pushers/set',
    data: data,
  );
  return;
}