taskmanager: Drop workaround for QTBUG-59044

The condition in ungrabMouseHack() is never satisfied, and the bug
should be fixed because QQuickWindow will get a mouseReleaseEvent in the
test case (http://static.davidedmundson.co.uk/qmlmousetest.tar.gz);
2 jobs for !841 with work/fusionfuture/QTBUG-59044 in 7 minutes and 52 seconds (queued for 2 seconds)
merge request