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

After commit bd33ef368135 ("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>

include/linux/page_ext.h

@@ -16,18 +16,7 @@ struct page_ext_operations {
 
 #ifdef CONFIG_PAGE_EXTENSION
 
-/*
- * page_ext->flags bits:
- *
- * PAGE_EXT_DEBUG_POISON is set for poisoned pages. This is used to
- * implement generic debug pagealloc feature. The pages are filled with
- * poison patterns and set this flag after free_pages(). The poisoned
- * pages are verified whether the patterns are not corrupted and clear
- * the flag before alloc_pages().
- */
-
 enum page_ext_flags {
-	PAGE_EXT_DEBUG_POISON,		/* Page is poisoned */
 	PAGE_EXT_DEBUG_GUARD,
 	PAGE_EXT_OWNER,
 #if defined(CONFIG_IDLE_PAGE_TRACKING) && !defined(CONFIG_64BIT)