LibWeb: Use [Reflect] to implement HTMLImageElement.decoding

This ensures the correct behavior when the value of the `decoding`
attribute is changed with `setAttribute()`.

Libraries/LibWeb/HTML/HTMLImageElement.h

@@ -87,10 +87,6 @@ public:
     // https://html.spec.whatwg.org/multipage/images.html#select-an-image-source
     [[nodiscard]] Optional<ImageSourceAndPixelDensity> select_an_image_source();
 
-    StringView decoding() const;
-
-    void set_decoding(String);
-
     void set_source_set(SourceSet);
 
     ImageRequest& current_request() { return *m_current_request; }
@@ -162,15 +158,6 @@ private:
     SourceSet m_source_set;
 
     CSSPixelSize m_last_seen_viewport_size;
-
-    // https://html.spec.whatwg.org/multipage/images.html#image-decoding-hint
-    enum class ImageDecodingHint {
-        Auto,
-        Sync,
-        Async
-    };
-
-    ImageDecodingHint m_decoding_hint = ImageDecodingHint::Auto;
 };
 
 }