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/DOM/Event.h

@@ -56,7 +56,7 @@ public:
         return Event::create(event_name, event_init);
     }
 
-    virtual ~Event() { }
+    virtual ~Event() = default;
 
     double time_stamp() const;