Services: Use default constructors/destructors

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/Services/NotificationServer/NotificationWindow.cpp

@@ -98,10 +98,6 @@ NotificationWindow::NotificationWindow(i32 client_id, const String& text, const
     };
 }
 
-NotificationWindow::~NotificationWindow()
-{
-}
-
 RefPtr<NotificationWindow> NotificationWindow::get_window_by_id(i32 id)
 {
     auto window = s_windows.get(id);