mm, page_flags: remove PG_slob_free

With SLOB removed we no longer need the PG_slob_free alias for
PG_private. Also update tools/mm/page-types.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Acked-by: Lorenzo Stoakes <lstoakes@gmail.com>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>