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/Taskbar/ShutdownDialog.cpp

@@ -117,7 +117,3 @@ ShutdownDialog::ShutdownDialog()
     // Request WindowServer to re-update us on the current theme as we might've not been alive for the last notification.
     refresh_system_theme();
 }
-
-ShutdownDialog::~ShutdownDialog()
-{
-}