mm/damon/sysfs: remove misleading todo comment in nid_show()

The TODO comment in nid_show() suggested returning an error if the goal
was not using nid. However, this comment was found to be inaccurate and
misleading.This patch removes the TODO comment without changing any
existing behavior.

This change follows feedback from SJ who pointed out [1] that wiring-order
independence is expected and the function should simply show the last set
value. and [2] checkpatch.pl complain about number of chars per line

No functional code changes were made.

Tested with KUnit:
- Built kernel with KUnit and DAMON sysfs tests enabled.
- Executed KUnit tests:
./tools/testing/kunit/kunit.py run --kunitconfig ./mm/damon/tests/
- All 25 tests passed, including damon_sysfs_test_add_targets.

Link: https://lkml.kernel.org/r/20251021215323.29734-2-swarajgaikwad1925@gmail.com
Link: https://lore.kernel.org/lkml/20251020151315.66260-1-sj@kernel.org/ [1]
Link: https://lore.kernel.org/lkml/20251021010847.68473-1-sj@kernel.org/ [2]
Signed-off-by: Swaraj Gaikwad <swarajgaikwad1925@gmail.com>
Suggested-by: SeongJae Park <sj@kernel.org>
Reviewed-by: SeongJae Park <sj@kernel.org>
Cc: David Hunter <david.hunter.linux@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>