YoutubeDL: Make the decision about removing the original file after each...

... postprocessor is run (fixes #2261)

If one of the processors said the file should be kept, it wouldn't pay
attention to the response from the following processors. This was wrong if the
'keep_video' option was False, if the first extractor modifies the original file
and then we extract its audio we don't want to keep the original video file.

I think that's how we should handle the `keep_video` option, but I want that someone else reviews this change.