mm: remove page_is_poisoned() from linux/mm.h

mm: remove page_is_poisoned() from linux/mm.h

When commit e530b02f9d7f ("mm: enable page poisoning early at boot") got
rid of the PAGE_EXT_DEBUG_POISON, page_is_poisoned in the header left
behind.  This patch cleans up the leftovers under the table.

Link: http://lkml.kernel.org/r/1528101069-21637-1-git-send-email-kpark3469@gmail.com
Signed-off-by: Sahara <keun-o.park@darkmatter.ae>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>