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/MediaList.h

@@ -20,7 +20,7 @@ public:
     {
         return adopt_ref(*new MediaList(move(media)));
     }
-    ~MediaList();
+    ~MediaList() = default;
 
     String media_text() const;
     void set_media_text(String const&);