block: remove obsolete config BLOCK_COMPAT

Before commit bdc1ddad

 ("compat_ioctl: block: move
blkdev_compat_ioctl() into ioctl.c"), the config BLOCK_COMPAT was used to
include compat_ioctl.c into the kernel build. With this commit, the code
is moved into ioctl.c and included with the config COMPAT. So, since then,
the config BLOCK_COMPAT has no effect and any further purpose.

Remove this obsolete config BLOCK_COMPAT.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20230316111630.4897-1-lukas.bulwahn@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>