Remove key from memory, leaving file in place.

        This forces :py:meth:`~bucketcache.buckets.DeferredWriteBucket.sync`.

def unload_key(self, key):
        """Remove key from memory, leaving file in place.

        This forces :py:meth:`~bucketcache.buckets.DeferredWriteBucket.sync`.
        """
        self.sync()
        return super(DeferredWriteBucket, self).unload_key(key)