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/CSSImportRule.h

@@ -26,7 +26,7 @@ public:
         return adopt_ref(*new CSSImportRule(move(url), document));
     }
 
-    ~CSSImportRule();
+    ~CSSImportRule() = default;
 
     const AK::URL& url() const { return m_url; }