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/CSS/StyleComputer.h

@@ -48,7 +48,7 @@ private:
 class StyleComputer {
 public:
     explicit StyleComputer(DOM::Document&);
-    ~StyleComputer();
+    ~StyleComputer() = default;
 
     DOM::Document& document() { return m_document; }
     DOM::Document const& document() const { return m_document; }