[SCSI] remove scsi_request infrastructure

With Achim patch the last user (gdth) is switched away from scsi_request
so we an kill it now.  Also disables some code in i2o_scsi that was
broken since the sg driver stopped using scsi_requests.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>