Cleanup: Cycles: Cleanup point cloud attribute update

For other geometry types this was done in bbe6d44928, but for some reason
point clouds were added with the old scheme.

intern/cycles/blender/pointcloud.cpp

@@ -220,10 +220,7 @@ void BlenderSync::sync_pointcloud(PointCloud *pointcloud, BObjectInfo &b_ob_info
     pointcloud->set_value(socket, new_pointcloud, socket);
   }
 
-  pointcloud->attributes.clear();
-  for (Attribute &attr : new_pointcloud.attributes.attributes) {
-    pointcloud->attributes.attributes.push_back(std::move(attr));
-  }
+  pointcloud->attributes.update(std::move(new_pointcloud.attributes));
 
   /* Tag update. */
   const bool rebuild = (pointcloud && old_numpoints != pointcloud->num_points());