LibCore: Stop atomically ref-counting EventReceiver

Inheriting from AtomicRefCounted and Weakable is asking for misuse of
WeakPtr to result in TOCTOU-caused UAFs.

In order to ensure we're not misusing EventReceiver across threads, I
ran test-web and some sites with a temporary hack to verify that ref()s
and unref()s are always called from the same thread on the class.

Libraries/LibCore/EventReceiver.h

@@ -37,7 +37,7 @@ public:                                            \
     }
 
 class CORE_API EventReceiver
-    : public AtomicRefCounted<EventReceiver>
+    : public RefCounted<EventReceiver>
     , public Weakable<EventReceiver> {
     // NOTE: No C_OBJECT macro for Core::EventReceiver itself.