turnip: delete tu_clear_sysmem_attachments_2d

turnip: delete tu_clear_sysmem_attachments_2d

2D path is using the same hardware as the 3D path, with the advantage of
separate register state, but here it requires WFI and extra cache flushing
and invalidating, so it should be better to just use the 3D path. There are
also some cases where the 3D path would be much faster, since it can clear
multiple attachments at once.

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5775>