memcg: remove unneeded forward declaration from sched.h

This forward declaration seems pointless.
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/sched.h

@@ -92,7 +92,6 @@ struct sched_param {
 
 #include <asm/processor.h>
 
-struct mem_cgroup;
 struct exec_domain;
 struct futex_pi_state;
 struct robust_list_head;