File attachments aren't deleted when removed from review requests

I tested delete files manually.

 1. I add s single file, then I delete it before publishing. The url r/xxxx/file/yyyy return 404.

 2. I add a single file, then I delete it after publishing. The file exists. 

 3. I add 2 files, one is add before publishing and another is add after publishing. The result is same as above.

Currently I am adding the unit test.

./reviewboard/manage.py test -- reviewboard.webapi.tests.test_file_attachment_draft