md/raid10: remove dead code in reshape_request

A bio allocated by bio_alloc_bioset comes pre-zeroed, no need to
clear random fields.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>