powerpc/powernv/npu: Remove atsd_threshold debugfs setting

This threshold is no longer used now that all invalidates issue a single
ATSD to each active NPU.

Signed-off-by: Mark Hairgrove <mhairgrove@nvidia.com>
Reviewed-by: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>