mm/memory-failure.c: remove useless "undef"s

Remove the useless #undef, since the corresponding #define has already
been removed.

Signed-off-by: Wang Xiaoqiang <wangxq10@lzu.edu.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>