memcg: remove mem_cgroup_reclaim_imbalance() remnants

commit 4f98a2fe (vmscan: split LRU lists
into anon & file sets) removed mem_cgroup_reclaim_imbalance(), but there
are some leftovers in memcontrol.h.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>