materio-base-d7/sites/all/modules/examples/block_example/block_example.install

<?php
/**
 * @file
 * Install, update and uninstall functions for the block_example module.
 */

/**
 * Implements hook_uninstall().
 *
 * @ingroup block_example
 */
function block_example_uninstall() {
  variable_del('block_example_string');
}