block: remove the unused blk_bounce enum

The enum has been replaced with the BLK_FEAT_BOUNCE_HIGH flag.

Reported-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20240619154623.450048-2-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>