blk-cgroup: Remove obsolete macro

Remove the obsolete 'MAX_KEY_LEN' macro.
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-cgroup.c

@@ -32,8 +32,6 @@
 #include <linux/psi.h>
 #include "blk.h"
 
-#define MAX_KEY_LEN 100
-
 /*
  * blkcg_pol_mutex protects blkcg_policy[] and policy [de]activation.
  * blkcg_pol_register_mutex nests outside of it and synchronizes entire