deleteUser method
This is used by an authenticated user to delete him/herself
Implementation
Future<bool> deleteUser() async {
  if (_signInUserSession == null || !_signInUserSession!.isValid()) {
    throw Exception('User is not authenticated');
  }
  final paramsReq = {
    'AccessToken': _signInUserSession!.getAccessToken().getJwtToken(),
  };
  await client!.request('DeleteUser', paramsReq);
  await clearCachedTokens();
  return true;
}