drop libsas task collector mode?

The task collector mode in libsas implements internal queueing in libsas, which is somethign we usualy try to avoid, and doesn't seem to get much exposure as it's only supported as a non-default option in two drivers.

If there's still interest in submitting multiple tasks in one go I'd be happy to wire up the blk-mq last parameter with a flag in the scsi command so that we could get a similar effect without all the overhead, but I'd need to have a signed up tester for it.

From the isci perspective I don't care about task_collector going