Bug 1791759 - remove unused ContentRelevancyReason::DescendantOfTopLayerElement r=emilio

A small cleanup, follow up from D159693.

Differential Revision: https://phabricator.services.mozilla.com/D169212

layout/base/LayoutConstants.h

@@ -96,9 +96,6 @@ enum class ContentRelevancyReason {
   // If the content of this Frame is on screen or nearly on screen.
   Visible,
 
-  // If this Frame's element is a descendant of a top layer element.
-  DescendantOfTopLayerElement,
-
   // If this Frame's element has focus in its subtree.
   FocusInSubtree,
 

layout/generic/nsIFrame.cpp

@@ -7039,13 +7039,6 @@ void nsIFrame::UpdateIsRelevantContent(
                       HasSelectionInSubtree());
   }
 
-  if (!oldRelevancy ||
-      aRelevancyToUpdate.contains(
-          ContentRelevancyReason::DescendantOfTopLayerElement)) {
-    setRelevancyValue(ContentRelevancyReason::DescendantOfTopLayerElement,
-                      IsDescendantOfTopLayerElement());
-  }
-
   bool overallRelevancyChanged =
       !oldRelevancy || oldRelevancy->isEmpty() != newRelevancy.isEmpty();
   if (!oldRelevancy || *oldRelevancy != newRelevancy) {