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/XHR/ProgressEvent.h

@@ -32,7 +32,7 @@ public:
         return ProgressEvent::create(event_name, event_init);
     }
 
-    virtual ~ProgressEvent() override { }
+    virtual ~ProgressEvent() override = default;
 
     bool length_computable() const { return m_length_computable; }
     u32 loaded() const { return m_loaded; }