Libraries: Use default constructors/destructors in LibCore

https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#cother-other-default-operation-rules

"The compiler is more likely to get the default semantics right and
you cannot implement these functions better than the compiler."

Userland/Libraries/LibCore/Event.cpp

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2022, the SerenityOS developers.
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -17,10 +18,6 @@ ChildEvent::ChildEvent(Type type, Object& child, Object* insertion_before_child)
 {
 }
 
-ChildEvent::~ChildEvent()
-{
-}
-
 Object* ChildEvent::child()
 {
     if (auto ref = m_child.strong_ref())