[WIP] Get rid of DownloadJob

Summary:
It doesn't do much and can be inlined into LanDeviceLink.

TODO: Figure out why sendfiletest fails

Test Plan: Receiving files still works

Reviewers: KDE Connect, albertvaka

Reviewed By: KDE Connect, albertvaka

Subscribers: albertvaka, kdeconnect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D14597