Restore event listening behavior in EntityOutlinerListModel (#4254)

Disables a problematic child reparenting code path in EditorEntityModel that led to the series of remove -> add -> remove notifications for reparented entities that led to unnecessary updates and a sporadic QSortFilterProxyModel crash

Signed-off-by: nvsickle <nvsickle@amazon.com>

Code/Framework/AzToolsFramework/AzToolsFramework/Entity/EditorEntityModel.cpp

@@ -381,12 +381,22 @@ namespace AzToolsFramework
             return;
         }
 
-        //orphan any children that remain attached to the entity
-        auto children = entityInfo.GetChildren();
-        for (auto childId : children)
+        bool isPrefabSystemEnabled = false;
+        AzFramework::ApplicationRequests::Bus::BroadcastResult(
+            isPrefabSystemEnabled, &AzFramework::ApplicationRequests::IsPrefabSystemEnabled);
+
+        // For slices, orphan any children that remain attached to the entity
+        // For prefabs, this is an unneeded operation because the prefab system handles the orphans
+        // and the extra reparenting operation can be problematic for consumers subscribed to entity
+        // events, such as the entity outliner.
+        if (!isPrefabSystemEnabled)
         {
-            ReparentChild(childId, AZ::EntityId(), entityId);
-            m_entityOrphanTable[entityId].insert(childId);
+            auto children = entityInfo.GetChildren();
+            for (auto childId : children)
+            {
+                ReparentChild(childId, AZ::EntityId(), entityId);
+                m_entityOrphanTable[entityId].insert(childId);
+            }
         }
 
         m_savedOrderInfo[entityId] = AZStd::make_pair(entityInfo.GetParent(), entityInfo.GetIndexForSorting());

Code/Framework/AzToolsFramework/AzToolsFramework/UI/Outliner/EntityOutlinerListModel.cpp

@@ -1344,7 +1344,10 @@ namespace AzToolsFramework
         //add/remove operations trigger selection change signals which assert and break undo/redo operations in progress in inspector etc.
         //so disallow selection updates until change is complete
         emit EnableSelectionUpdates(false);
-        beginResetModel();
+
+        auto parentIndex = GetIndexFromEntity(parentId);
+        auto childIndex = GetIndexFromEntity(childId);
+        beginRemoveRows(parentIndex, childIndex.row(), childIndex.row());
     }
 
     void EntityOutlinerListModel::OnEntityInfoUpdatedRemoveChildEnd(AZ::EntityId parentId, AZ::EntityId childId)
@@ -1352,7 +1355,7 @@ namespace AzToolsFramework
         (void)childId;
         AZ_PROFILE_FUNCTION(AzToolsFramework);
 
-        endResetModel();
+        endRemoveRows();
 
         //must refresh partial lock/visibility of parents
         m_isFilterDirty = true;