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>