mm/mm_init: remove unnecessary initialization of pgdat->per_cpu_nodestats

free_area_init_node() sets pgdat->per_cpu_nodestats to NULL and later
calls free_area_init_core(), which unconditionally overwrites it with
&boot_nodestats. Nothing reads the field in between, so the store has
no effect.

Remove unnecessary initialization. No functional change.

Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>
Link: https://patch.msgid.link/20260729155143.177790-1-ekffu200098@gmail.com
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>