No need to use a QBuffer in DownloadJob

Summary:
QSslSocket is already a QIODevice so just use that.

Should fix the issue of transfering *big* files.

Unfortunately this seems to trigger a bug in KIO and CPU usage goes through the roof, so haven't really been able to test it does actually fix things.

Please don't merge/approve yet

Reviewers: apol, albertvaka, kdeconnect

Subscribers: KDE Connect

Differential Revision: https://phabricator.kde.org/D6039