Remove remains of old FileDownloader class, which is now redundant

The `youtube_dl/FileDownloader` says: `# Legacy file for backwards compatibility, use youtube_dl.downloader instead!`

So I checked for usages of this class, and it turns out the only place it is used is in  `youtube_dl/__init__.py`.
Actually, the only method used is `FileDownloader.parse_bytes`, which is defined as a static method in `.downloader/FileDownloader` anyway!
The old `FileDownloader._do_download` is never used.
To summarize, the class can safely be deleted (unless my IDE and I missed something).