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/AbortSignal.h

@@ -37,7 +37,7 @@ public:
         return AbortSignal::create();
     }
 
-    virtual ~AbortSignal() override;
+    virtual ~AbortSignal() override = default;
 
     void add_abort_algorithm(Function<void()>);