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.h

@@ -16,7 +16,7 @@ class Compositor;
 class WindowStack {
 public:
     WindowStack(unsigned row, unsigned column);
-    ~WindowStack();
+    ~WindowStack() = default;
 
     bool is_empty() const { return m_windows.is_empty(); }
     void add(Window&);