Don't extract uploaded dataset archive when conversion is necessary, anyway

[philippotto]

When uploading a dataset as a zip, the back-end currently unzips the archive and then delegates to the worker for conversion. However, the worker should do the unpacking (less load for the back-end and if this fails, we have a worker run in our system which we can work with to fix this).

If a dataset doesn't need to be converted, the back-end should do the unzipping as before.