util: Don't use deprecated GLib.IOSchedulerJob API

Co-author: Luca Bruno <lucabru@src.gnome.org>
util: Remove unused Cancellable arg of run_in_thread

Its not used so it only gives false hope to callers. If caller needs
cancellable, they can act on it on their own when in the passed
callback.
downloader: Don't use deprecated libsoup API