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/CSSRuleList.h

@@ -27,7 +27,7 @@ public:
     {
         return adopt_ref(*new CSSRuleList(move(rules)));
     }
-    ~CSSRuleList();
+    ~CSSRuleList() = default;
 
     RefPtr<CSSRule> item(size_t index) const
     {