You could add it at the top of user/edit_form.php, next to the block of code that starts with the comment 'Add some extra hidden fields'. But then those fields would be added again in user/editlib.php giving you a nice error.
So you can edit user/editlib.php and search for the block of lines that read:
$mform->addElement('text', 'city', get_string('city'), 'maxlength="20" size="21"'); $mform->setType('city', PARAM_MULTILANG); $mform->addRule('city', $strrequired, 'required', null, 'client');
and simply comment the last line (the addRule line). A few lines below you'll find the code for the country field. Just do the same.