include/linux/page_ext.h: drop definition of unused PAGE_EXT_DEBUG_POISON

After commit bd33ef36

 ("mm: enable page poisoning early at boot")
PAGE_EXT_DEBUG_POISON is not longer used.  Remove it.

Link: http://lkml.kernel.org/r/20180531135457.20167-2-kirill.shutemov@linux.intel.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Vinayak Menon <vinmenon@codeaurora.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>