[SCSI] remove requeue feature from blk_insert_request()

blk_insert_request() has a unobivous feature of requeuing a
request setting REQ_SPECIAL|REQ_SOFTBARRIER.  SCSI midlayer
was the only user and as previous patches removed the usage,
remove the feature from blk_insert_request().  Only special
requests should be queued with blk_insert_request().  All
requeueing should go through blk_requeue_request().

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>