LibCore: Remove data pointer from CustomEvent

It wasn't used anywhere.

Also, if it were used, then it should have been marked AK_NONCOPYABLE().
Or even more cleanly, it should use a RefPtr<> or OwnPtr<> instead of
a 'naked' pointer. And because I didn't want to impose any such decision
on a possible future use case that we don't even know, I just removed
that unused feature.

Libraries/LibCore/Event.h

@@ -143,21 +143,17 @@ private:
 
 class CustomEvent : public Event {
 public:
-    CustomEvent(int custom_type, void* data = nullptr)
+    CustomEvent(int custom_type)
         : Event(Event::Type::Custom)
         , m_custom_type(custom_type)
-        , m_data(data)
     {
     }
     ~CustomEvent() { }
 
     int custom_type() const { return m_custom_type; }
-    void* data() { return m_data; }
-    const void* data() const { return m_data; }
 
 private:
     int m_custom_type { 0 };
-    void* m_data { nullptr };
 };
 
 }