Remove the win32 file watcher

Summary:
Our changes have almost certainly broken the win32 file watcher, which we can't
easily test or fix. But we don't really care because we don't support
developing on Windows anyway.

Test Plan: `ack --python win32_file_watcher`

Reviewers: csilvers

Reviewed By: csilvers

Differential Revision: http://phabricator.khanacademy.org/D9185