code: synchronized void removeCredential(String accountId) throws IOException {
          DataStore<StoredCredential> sc = StoredCredential.getDefaultDataStore(dataStoreFactory);
          sc.delete(accountId);
          calendarService = null;
          geoService = null;
      }
docs: Deletes the stored credentials for the given account id. This means the
  next time the user must authorize the app to access his calendars.
  
  @param accountId
  The identifier of the account.