Provide missing uninstall code

Following up here based on a better understanding of how fields work.
A helpful post here describing the same issue (but with different modules) can be found here http://drupal.org/node/1284332
More specifically this is the recommended approach Field types should be defined by one module and implemented by a separate module.

Notes on how to disable the module:

1. back up your db
2. remove the addressbook field from the field_config table
3. remove the addressbook field instance from the field_config_instance table
4. disable the module.
5. remove the field_data_addressbook_saved_profiles table (optional as this destroys data created by the module)
6. remove the field_revision_addressbook_saved_profiles table (optional as this destroys data created by the module)
7. remove the commerce_addressbook_auto_prefill row from the variable table (optional as this destroys data created by the module)

Suggestion: implement a simple uninstall function which integrates into the development of the module(s).

/**
 * Implements hook_uninstall().
 *
 */
function commerce_addressbook_uninstall() {
  // Delete vars
    variable_del('commerce_addressbook_auto_prefill');

  // TODO: Delete fields. this is just a placeholder and will not function at present
/*  foreach (array_keys(_commerce_addressbook_installed_fields()) as $field) {
    field_delete_field($field);
  }
*/

  // TODO: Delete field instances. this is just a placeholder and will not function at present
/*  $instances = field_info_instances('commerce_addressbook_saved_profiles', 'addressbook_saved_profiles');
  foreach ($instances as $instance_name => $instance) {
    field_delete_instance($instance);
  }
*/

  // Purge all remaining field info.
  field_purge_batch(1000);

}