blkcg: kill unused field nr_undestroyed_grps

'nr_undestroyed_grps' in struct throtl_data was used to count
the number of throtl_grp related with throtl_data, but now
throtl_grp is tracked by blkcg_gq, so it is useless anymore.
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-throttle.c

@@ -145,11 +145,6 @@ struct throtl_data
 	/* Total Number of queued bios on READ and WRITE lists */
 	unsigned int nr_queued[2];
 
-	/*
-	 * number of total undestroyed groups
-	 */
-	unsigned int nr_undestroyed_grps;
-
 	/* Work for dispatching throttled bios */
 	struct work_struct dispatch_work;
 };