block: Remove request.write_hint

Commit c75e707f

 ("block: remove the per-bio/request write hint")
removed all code that uses the struct request write_hint member. Hence
also remove 'write_hint' itself.

Reviewed-by: Ming Lei <ming.lei@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20221025191755.1711437-2-bvanassche@acm.org
Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>