selftests/mm: remove unnecessary ia64 code and comment

IA64 has gone with commit cf8e8658 ("arch: Remove Itanium (IA-64)
architecture"), so remove unnecessary ia64 special mm code and comment in
selftests too.

Link: https://lkml.kernel.org/r/20240819130609.3386195-1-tujinjiang@huawei.com


Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nanyong Sun <sunnanyong@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>