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/WindowServer/WindowStack.cpp

@@ -15,10 +15,6 @@ WindowStack::WindowStack(unsigned row, unsigned column)
 {
 }
 
-WindowStack::~WindowStack()
-{
-}
-
 void WindowStack::add(Window& window)
 {
     VERIFY(!window.is_on_any_window_stack({}));