hosting_subdirs fails to clean up subdir.d folder when sites are deleted

Per my comment over on #3066146, the subdir.d folder (using either Apache or Nginx) doesn't get cleaned up when a subdir site is removed. The subdir.conf file for the deleted site should be removed.

@ergonlogic suggests:

* that'd be a post_delete drush/provision hook, something like drush_subdirs_post_provision_delete()
* maybe take a look at drush_http_provision_delete()
* the base class that's responsible for the subdir vhost provides an unlink() method

Which sounds like a good way to go to me :)