libata: remove unused ata_sas_port_async_resume() stub

Commit bc6e7c4b "libata, libsas: kill pm_result and related cleanup"
renamed ata_sas_port_async_resume() to ata_sas_port_resume(), but missed
a CONFIG_PM=n stub conversion.  Randy fixed that up in commit
a5a65699 "libata.h: add stub for ata_sas_port_resume", but missed
the deletion of the now unused ata_sas_port_async_resume() routine.

Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>