update_delete_file_if_stale() must return a value or update_manager_file_get() will always fetch remote files

  if (!file_exists($local) || update_delete_file_if_stale($local)) {
    return system_retrieve_file($url, $local, FALSE, FILE_EXISTS_REPLACE);
  }
  else {
    return $local;
  }

We'll never use the local cache of a remote file since update_delete_file_if_stale() is a void function.

The patch should:

• change update_delete_file_if_stale() to return TRUE if something is deleted and FALSE if not.
• update UpdateDeleteFileIfStaleTest to test the return value.