block: simplify sb_is_blkdev_sb

Just use IS_ENABLED instead of providing a stub for !CONFIG_BLOCK.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>