Revert "mm: Ensure that pfn_valid() is called once per pageblock when reserving pageblocks setup_zone_migrate_reserve() expects that zone->start_pfn starts at pageblock_nr_pages aligned pfn otherwise we could access beyond an existing memblock resulting in the following panic if CONFIG_HOLES_IN_ZONE is not configured and we do not check pfn_valid:"

This reverts commit bd5afcad14407d0460a12a4b8ccce3e97b1ca36e.