blk-wbt: remove unnecessary check in wbt_enable_default()

If CONFIG_BLK_WBT_MQ is disabled, wbt_init() won't do anything.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20221019121518.3865235-3-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>