Remove download properly on profile destruction to avoid use after free

In the Widgets API, download items are children of the profile and are
destroyed when the parent profile destroys its children. The download
item's destructor can therefore not access the profile, as it would
cause a heap-use-after-free crashes. On quick side turn ongoing downloads
cleanup to match widgets one.

Fixes: QTBUG-73839
Change-Id: Iabb379e91187e3e68ebcd4693fec35883b72b1f2
Reviewed-by: Michael Brüning <michael.bruning@qt.io>
Reviewed-by: Jüri Valdmann <juri.valdmann@qt.io>