rbd: remove the discard_zeroes_data flag

rbd only supports discarding on large alignments, so the zeroing code
would always fall back to explicit writings of zeroes.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Jens Axboe <axboe@fb.com>