Data of new account form validates in file
public_html/storefront/model/account/customer.php method validateRegistrationData, line #174
you can remove check for city
if ((strlen(utf8_decode($data['city'])) < 3) || (strlen(utf8_decode($data['city'])) > 128)) {
$error['city'] = $this->language->get('error_city');
}
or just comment.
Also you can open file public_html/storefront/controller/pages/account/create.php
and change 'required' => true by 'required' => false
$this->data['form'][ 'city' ] = $form->getFieldHtml( array(
'type' => 'input',
'name' => 'city',
'value' => $this->request->post['city'],
'required' => true ));