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.

Note: this will be useful for D32_S8 (won't need to deal with interaction with tu_clear_sysmem_attachments_2d), and also help expose problems with the WIP LRZ implementation