blk-iolatency: remove set but not used variables 'changed' and 'blkiolat'

Fixes gcc '-Wunused-but-set-variable' warning:

block/blk-iolatency.c: In function 'scale_change':
block/blk-iolatency.c:301:7: warning:
 variable 'changed' set but not used [-Wunused-but-set-variable]

block/blk-iolatency.c: In function 'iolatency_set_limit':
block/blk-iolatency.c:765:24: warning:
 variable 'blkiolat' set but not used [-Wunused-but-set-variable]

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>