Bug 1897749 - Remove unused HadWillChange(), r=tnikkel

Differential Revision: https://phabricator.services.mozilla.com/D210905

layout/painting/RetainedDisplayListBuilder.h

@@ -60,10 +60,6 @@ struct RetainedDisplayListData {
     return GetFlags(aFrame).contains(FrameFlag::HasProps);
   }
 
-  bool HadWillChange(nsIFrame* aFrame) const {
-    return GetFlags(aFrame).contains(FrameFlag::HadWillChange);
-  }
-
   /**
    * Returns an iterator to the underlying frame storage.
    */