direct-io: remove random prefetches

Randomly poking into block device internals for manual prefetches isn't
exactly a very maintainable thing to do.  And none of the performance
critical direct I/O implementations still use this library function
anyway, so just drop it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Link: https://lore.kernel.org/r/20220415045258.199825-28-hch@lst.de


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