isci: kill smp_discover_response

An lldd need never look at the contents of an smp_discover_response frame.
Kill the remaining locations where isci is looking at it:

1/ covering for expanders that do not set the stp_attached bit (already
   handled by sas_ex_discover_end_dev)
2/ an overkill method to notifiy the rest of the driver about remote_device
   sas addresses

Signed-off-by: Dan Williams <dan.j.williams@intel.com>