Revert "block: implement blkdev_readpages"

This reverts commit db2dbb12dc47a50c7a4c5678f526014063e486f6.

It apparently causes problems with partition table read-ahead
on archs with large page sizes. Until that problem is diagnosed
further, just drop the readpages support on block devices.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>