Since anki.media.MediaManager does not store per-file usn (as it's
not really needed for anything in the client), this requires us to
drop it and implement a custom media manager.
A class which keeps track of temporary files and removes them
automatically when they're not needed anymore might be a good idea,
but this implementation didn't remove files in some cases. Adding
unrelated methods that could as well be just standalone functions
is bad design, too.
In this case, it's better to just get rid of it altogether instead
of fixing it, since Python 3 has a TemporaryDirectory class, which
can be used for the same purpose and is definitely more
battle-tested.
