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.cpp

@@ -28,10 +28,6 @@ CSSImportRule::CSSImportRule(AK::URL url, DOM::Document& document)
     set_resource(ResourceLoader::the().load_resource(Resource::Type::Generic, request));
 }
 
-CSSImportRule::~CSSImportRule()
-{
-}
-
 // https://www.w3.org/TR/cssom/#serialize-a-css-rule
 String CSSImportRule::serialized() const
 {