mm: page_alloc: remove remnants of unlocked migratetype updates

The freelist hygiene patches made migratetype accesses fully protected
under the zone->lock.  Remove remnants of handling the race conditions
that existed before from the MIGRATE_HIGHATOMIC code.

Link: https://lkml.kernel.org/r/20250225001023.1494422-3-hannes@cmpxchg.org


Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>