LibThreading: Remove Thread's inheritance from Core::EventReceiver

Inheritance from `EventReceiver` on the `Thread` class was only used in
the `BackgroundAction` class, where the children vector was keeping the
action alive until the work was completed. However, this can be
accomplished by instead capturing a `NonnullRefPtr` of `this`. The work
function can then avoid having to remove the `BackgroundAction` from
its parent `Thread` when the work completes.

Userland/Libraries/LibThreading/Thread.cpp

@@ -12,8 +12,7 @@
 namespace Threading {
 
 Thread::Thread(Function<intptr_t()> action, StringView thread_name)
-    : Core::EventReceiver(nullptr)
-    , m_action(move(action))
+    : m_action(move(action))
     , m_thread_name(thread_name.is_null() ? ""sv : thread_name)
 {
 }