LibWeb: Remove unused VideoBox preferred width/height methods

These were copied from some other paintable, but were never used for
video.

Userland/Libraries/LibWeb/Layout/VideoBox.h

@@ -33,9 +33,6 @@ private:
 
     // ^JS::Cell
     virtual void finalize() override;
-
-    int preferred_width() const;
-    int preferred_height() const;
 };
 
 }