The grid layout for UI lists wasn't used in practice from all we can
tell. It was badly maintained for a long time (bugs went unnoticed). I
think it was added for an earlier version of the asset UI design.
This was planned for removal in 5.0, see blender/blender#110461.
Usages in bundled scripts were already removed in efa8d942b8.
Pull Request: https://projects.blender.org/blender/blender/pulls/146656