remove the sx8 block driver

This driver is for fairly obscure hardware, and has only seen random
drive-by changes after the maintainer stopped working on it in 2005
(about a year and a half after it was introduced).  It has some
"interesting" block layer interactions, so let's just drop it unless
anyone complains.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220721064102.1715460-1-hch@lst.de
[axboe: fix date typo, it was in 2005, not 2015]
Signed-off-by: Jens Axboe <axboe@kernel.dk>