Removing instance settings on uninstall

gistfile1.php

<?php

/**
 * Implements hook_uninstall().
 */
function mymodule_uninstall() {
  $fields = field_read_fields(array('module' => 'mymodule'));
  $instances = field_read_instances(array('field_name' => array_keys($fields)));
  foreach ($instances as $instance) {
    if (isset($instance['settings']['my_added_setting'])) {
      unset($instance['settings']['my_added_setting']);
      field_update_instance($instance);
    }
  }
}