progress-info: Prevent crashes when job is cancelled

In order to prevent deadlock when cancelling operations, new thread were added by commit 2db9a295. However, this introduce various races. One of them has been fixed by commit 45c7c967. However, I am convinced that there is still a race when finalizing. One way to fix it is the usage of g_thread_join. But I think that it would be better to avoid the deadlock another way. Let's instead remove the problematic thread and call g_cancellable_cancel without the lock held.

Fixes: #124 (closed)