include/linux/mmzone.h: remove unused is_unevictable_lru()

Since commit a0b8cab3 ("mm: remove lru parameter from
__pagevec_lru_add and remove parts of pagevec API") there's no
user of this function anymore, so remove it.

Signed-off-by: Yaowei Bai <baiyaowei@cmss.chinamobile.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>