Delete fields in hook_update_n

/**
 * Delete fields no longer used by this module.
 */
function hook_update_N() {
  $fields_to_delete = array(
    'field_cost',
    'field_construction_budget',
    'field_construction_cost',
    'field_total_budget',
  );
  foreach ($fields_to_delete as $field_name) {
    field_delete_field($field_name);
    watchdog('module_name', 'Deleted the :field_name field from all content type instances.', array(':field_name' => $field_name));
  }

  /**
   * The fields aren't really deleted until the purge function runs, ordinarily
   * during cron.  Count the number of fields we need to purge, and add five in
   * case a few other miscellaneous fields are in there somehow.
   */
  field_purge_batch(count($fields_to_delete) + 5);

  // THEN, in case of field type change, revert the feature
  features_revert(array('module_name' => array('field')));
}