LibThread: Fix destroying background actions

In the old model, before bc319d9, the parent
(the background thread) would delete us when it exits (i.e. never), so we had to
keep track of our own refcount in order to destroy ourselves when we're done.

With bc319d9, the parent keeps additional
reference to us, so:
* There should be no need to explicitly ref() ourselves
* The unref() would not get rid of the last reference to us anymore

The latter is why all the BackgroundAction's were getting leaked. Fix this by
simply unparenting ourselves from the background thread when we're done.

Libraries/LibThread/BackgroundAction.h

@@ -75,17 +75,17 @@ class BackgroundAction final : public Core::Object
  {
  LOCKER(all_actions().lock());
 
- this->ref();
  all_actions().resource().enqueue([this] {
  m_result = m_action();
  if (m_on_complete) {
  Core::EventLoop::current().post_event(*this, make<Core::DeferredInvocationEvent>([this](auto&) {
  m_on_complete(m_result.release_value());
- this->unref();
+ this->remove_from_parent();
  }));
  Core::EventLoop::wake();
- } else
- this->unref();
+ } else {
+ this->remove_from_parent();
+ }
  });
  }