memcg: remove lookup_cgroup_page() prototype

Commit 6b208e3f ("mm: memcg: remove unused node/section info from
pc->flags") deleted lookup_cgroup_page() but left a prototype for it.

Kill the vestigial prototype.
Signed-off-by: Greg Thelen <gthelen@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/page_cgroup.h

@@ -40,7 +40,6 @@ static inline void page_cgroup_init(void)
 #endif
 
 struct page_cgroup *lookup_page_cgroup(struct page *page);
-struct page *lookup_cgroup_page(struct page_cgroup *pc);
 
 static inline int PageCgroupUsed(struct page_cgroup *pc)
 {