Libraries: Use default constructors/destructors in LibWeb

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/LibWeb/HTML/Parser/StackOfOpenElements.h

@@ -20,7 +20,7 @@ public:
     // and the bottommost node of the stack is the most recently added node in the stack
     // (notwithstanding when the stack is manipulated in a random access fashion as part of the handling for misnested tags).
 
-    StackOfOpenElements() { }
+    StackOfOpenElements() = default;
     ~StackOfOpenElements();
 
     DOM::Element& first() { return m_elements.first(); }